JIT: fix incorrect scale in genCreateAddrMode + no-opt

src/coreclr/jit/codegencommon.cpp

@@ -1389,6 +1389,7 @@ bool CodeGen::genCreateAddrMode(
         {
             ssize_t  tmpMul;
             GenTree* index;
+            bool     indexIsEffectivelyZero = false;
 
             if ((rv2->gtOper == GT_MUL || rv2->gtOper == GT_LSH) && (rv2->AsOp()->gtOp2->IsCnsIntOrI()))
             {
@@ -1401,6 +1402,12 @@ bool CodeGen::genCreateAddrMode(
                 {
                     tmpMul *= mul;
                 }
+
+                if (tmpMul == 0)
+                {
+                    // "Index * 0" (if it wasn't folded earlier) means the index is zero
+                    indexIsEffectivelyZero = true;
+                }
             }
             else
             {
@@ -1410,8 +1417,13 @@ bool CodeGen::genCreateAddrMode(
                 tmpMul = mul;
             }
 
+            if (indexIsEffectivelyZero)
+            {
+                mul = 0;
+                rv2 = nullptr;
+            }
             /* Get hold of the array index and see if it's a constant */
-            if (index->IsIntCnsFitsInI32())
+            else if (index->IsIntCnsFitsInI32())
             {
                 /* Get hold of the index value */
                 ssize_t ixv = index->AsIntConCommon()->IconValue();

src/tests/JIT/Regression/JitBlue/Runtime_75312/Runtime_75312.il

@@ -0,0 +1,52 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+.assembly extern System.Runtime {}
+.assembly extern System.Runtime.Extensions {}
+.assembly extern System.Console {}
+.assembly Runtime_75312 {}
+
+.class public abstract auto ansi sealed beforefieldinit Runtime_75312
+       extends [System.Runtime]System.Object
+{
+    .method private hidebysig static 
+        int32 Main () cil managed 
+    {
+        .entrypoint
+        .maxstack 2
+        .locals init (
+            [0] int64 a
+        )
+        ldc.i8 1234605616436508552
+        stloc.0
+        ldc.i4 1146447579
+        ldloca.s 0
+        conv.u
+        newobj instance void [System.Runtime]System.IntPtr::.ctor(void*)
+        call int32 [System.Runtime]System.IntPtr::op_Explicit(native int)
+        call int32 Runtime_75312::Test(int32)
+        sub
+        ret
+    }
+
+    .method private hidebysig static int32 Test (int32 lcl) cil managed noinlining nooptimization
+    {
+        .maxstack 8
+
+        // return *(int*)(arg0 + ((3 * 0) << 2) + 1);
+        // to avoid constant folding in Roslyn (even for Debug) it's written in IL
+
+        ldarg.0
+        ldc.i4.3
+        ldc.i4.0
+        mul
+        ldc.i4.2
+        shl
+        add
+        ldc.i4.1
+        add
+        conv.i
+        ldind.i4
+        ret
+    }
+}

src/tests/JIT/Regression/JitBlue/Runtime_75312/Runtime_75312.ilproj

@@ -0,0 +1,11 @@
+<Project Sdk="Microsoft.NET.Sdk.IL">
+  <PropertyGroup>
+    <OutputType>Exe</OutputType>
+    <CLRTestTargetUnsupported Condition="'$(TargetBits)' != '32'">true</CLRTestTargetUnsupported>
+    <DebugType>None</DebugType>
+    <Optimize>True</Optimize>
+  </PropertyGroup>
+  <ItemGroup>
+    <Compile Include="$(MSBuildProjectName).il" />
+  </ItemGroup>
+</Project>