[Mono] Restore old code to solve the recent SpanHelpers regressions

src/libraries/System.Private.CoreLib/src/System.Private.CoreLib.Shared.projitems

@@ -2462,4 +2462,10 @@
     <Compile Include="$(MSBuildThisFileDirectory)System\Numerics\IUnaryPlusOperators.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)System\Numerics\IUnsignedNumber.cs" />
   </ItemGroup>
+  <ItemGroup Condition="'$(FeatureCoreCLR)' == 'true'">
+    <Compile Include="$(MSBuildThisFileDirectory)System\SpanHelpers.Clr.cs" />
+  </ItemGroup>
+  <ItemGroup Condition="'$(FeatureMono)' == 'true'">
+    <Compile Include="$(MSBuildThisFileDirectory)System\SpanHelpers.Mono.cs" />
+  </ItemGroup>
 </Project>

src/libraries/System.Private.CoreLib/src/System/SpanHelpers.Clr.cs

@@ -0,0 +1,82 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using System.Collections.Generic;
+using System.Diagnostics;
+using System.Numerics;
+using System.Runtime.CompilerServices;
+using System.Runtime.Intrinsics;
+
+namespace System
+{
+    internal static partial class SpanHelpers // helpers used by CLR
+    {
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        internal static int IndexOfChar(ref char searchSpace, char value, int length)
+            => IndexOfValueType(ref Unsafe.As<char, short>(ref searchSpace), (short)value, length);
+
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        internal static int IndexOfValueType<T>(ref T searchSpace, T value, int length) where T : struct, INumber<T>
+            => IndexOfValueType<T, DontNegate<T>>(ref searchSpace, value, length);
+
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        internal static int IndexOfAnyExceptValueType<T>(ref T searchSpace, T value, int length) where T : struct, INumber<T>
+            => IndexOfValueType<T, Negate<T>>(ref searchSpace, value, length);
+
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        internal static int LastIndexOfValueType<T>(ref T searchSpace, T value, int length) where T : struct, INumber<T>
+            => LastIndexOfValueType<T, DontNegate<T>>(ref searchSpace, value, length);
+
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        internal static int LastIndexOfAnyExceptValueType<T>(ref T searchSpace, T value, int length) where T : struct, INumber<T>
+            => LastIndexOfValueType<T, Negate<T>>(ref searchSpace, value, length);
+
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        internal static int IndexOfAnyChar(ref char searchSpace, char value0, char value1, int length)
+            => IndexOfAnyValueType(ref Unsafe.As<char, short>(ref searchSpace), (short)value0, (short)value1, length);
+
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        internal static int IndexOfAnyValueType<T>(ref T searchSpace, T value0, T value1, int length) where T : struct, INumber<T>
+            => IndexOfAnyValueType<T, DontNegate<T>>(ref searchSpace, value0, value1, length);
+
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        internal static int IndexOfAnyExceptValueType<T>(ref T searchSpace, T value0, T value1, int length) where T : struct, INumber<T>
+            => IndexOfAnyValueType<T, Negate<T>>(ref searchSpace, value0, value1, length);
+
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        internal static int LastIndexOfAnyValueType<T>(ref T searchSpace, T value0, T value1, int length) where T : struct, INumber<T>
+            => LastIndexOfAnyValueType<T, DontNegate<T>>(ref searchSpace, value0, value1, length);
+
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        internal static int LastIndexOfAnyExceptValueType<T>(ref T searchSpace, T value0, T value1, int length) where T : struct, INumber<T>
+            => LastIndexOfAnyValueType<T, Negate<T>>(ref searchSpace, value0, value1, length);
+
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        internal static int IndexOfAnyValueType<T>(ref T searchSpace, T value0, T value1, T value2, int length) where T : struct, INumber<T>
+            => IndexOfAnyValueType<T, DontNegate<T>>(ref searchSpace, value0, value1, value2, length);
+
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        internal static int IndexOfAnyExceptValueType<T>(ref T searchSpace, T value0, T value1, T value2, int length) where T : struct, INumber<T>
+            => IndexOfAnyValueType<T, Negate<T>>(ref searchSpace, value0, value1, value2, length);
+
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        internal static int LastIndexOfAnyValueType<T>(ref T searchSpace, T value0, T value1, T value2, int length) where T : struct, INumber<T>
+            => LastIndexOfAnyValueType<T, DontNegate<T>>(ref searchSpace, value0, value1, value2, length);
+
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        internal static int LastIndexOfAnyExceptValueType<T>(ref T searchSpace, T value0, T value1, T value2, int length) where T : struct, INumber<T>
+            => LastIndexOfAnyValueType<T, Negate<T>>(ref searchSpace, value0, value1, value2, length);
+
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        internal static int IndexOfAnyValueType<T>(ref T searchSpace, T value0, T value1, T value2, T value3, int length) where T : struct, INumber<T>
+            => IndexOfAnyValueType<T, DontNegate<T>>(ref searchSpace, value0, value1, value2, value3, length);
+
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        internal static int IndexOfAnyExceptValueType<T>(ref T searchSpace, T value0, T value1, T value2, T value3, int length) where T : struct, INumber<T>
+            => IndexOfAnyValueType<T, Negate<T>>(ref searchSpace, value0, value1, value2, value3, length);
+
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        internal static int LastIndexOfAnyExceptValueType<T>(ref T searchSpace, T value0, T value1, T value2, T value3, int length) where T : struct, INumber<T>
+            => LastIndexOfAnyValueType<T, Negate<T>>(ref searchSpace, value0, value1, value2, value3, length);
+    }
+}