Fix bug in String.Equals unrolling for certain single-char strings (#78234)

Author: EgorBo

src/coreclr/jit/importer_vectorization.cpp

@@ -380,7 +380,7 @@ GenTree* Compiler::impExpandHalfConstEqualsSWAR(
         //   [ ch1 ]
         //   [value]
         //
-        return impCreateCompareInd(data, TYP_SHORT, dataOffset, cns[0], cmpMode);
+        return impCreateCompareInd(data, TYP_USHORT, dataOffset, cns[0], cmpMode);
     }
     if (len == 2)
     {

src/tests/JIT/opt/Vectorization/StringEquals.cs

@@ -22,7 +22,7 @@ public static int Main()
         }
 
         Console.WriteLine(testCount);
-        return testCount == 25920 ? 100 : 0;
+        return testCount == 27888 ? 100 : 0;
     }
 }
 
@@ -195,10 +195,22 @@ static void ValidateEquals(bool result, string left, string right)
     [MethodImpl(MethodImplOptions.NoInlining)] public static void Equals_157(string s) => ValidateEquals(s == "2ж1311a23b2212aЙ21Й11жb3233bb3a1", s, "2ж1311a23b2212aЙ21Й11жb3233bb3a1");
     [MethodImpl(MethodImplOptions.NoInlining)] public static void Equals_158(string s) => ValidateEquals(s == "01Й11113ь3Й32a3ьЙЙ3Й32b2ab221310", s, "01Й11113ь3Й32a3ьЙЙ3Й32b2ab221310");
     [MethodImpl(MethodImplOptions.NoInlining)] public static void Equals_159(string s) => ValidateEquals(s == "a120213b11211\0223223312ьь1Й3222Й", s, "a120213b11211\0223223312ьь1Й3222Й");
+    [MethodImpl(MethodImplOptions.NoInlining)] public static void Equals_160(string s) => ValidateEquals(s == "\u9244", s, "\u9244");
+    [MethodImpl(MethodImplOptions.NoInlining)] public static void Equals_161(string s) => ValidateEquals(s == "\u9244\u9244", s, "\u9244\u9244");
+    [MethodImpl(MethodImplOptions.NoInlining)] public static void Equals_162(string s) => ValidateEquals(s == "\u9244\u9244\u9244", s, "\u9244\u9244\u9244");
+    [MethodImpl(MethodImplOptions.NoInlining)] public static void Equals_163(string s) => ValidateEquals(s == "\uFFFF", s, "\uFFFF");
+    [MethodImpl(MethodImplOptions.NoInlining)] public static void Equals_164(string s) => ValidateEquals(s == "\uFFFF\uFFFF", s, "\uFFFF\uFFFF");
+    [MethodImpl(MethodImplOptions.NoInlining)] public static void Equals_165(string s) => ValidateEquals(s == "\uFFFF\uFFFF\uFFFF", s, "\uFFFF\uFFFF\uFFFF");
 
     public static readonly string[] s_TestData =
     {
         null,
+        "\u9244",
+        "\u9244\u9244",
+        "\u9244\u9244\u9244",
+        "\uFFFF",
+        "\uFFFF\uFFFF",
+        "\uFFFF\uFFFF\uFFFF",
         "",
         "\0",
         "a",

src/tests/JIT/opt/Vectorization/UnrollEqualsStartsWIth.cs

@@ -18,7 +18,7 @@ public static int Main()
         int testCount = 0;
         foreach (var testType in testTypes)
             testCount += RunTests(testType);
-        return testCount == 113652 ? 100 : 0;
+        return testCount == 127512 ? 100 : 0;
     }
 
     public static int RunTests(Type type)
@@ -37,6 +37,12 @@ public static int RunTests(Type type)
 
         string[] testData =
         {
+            "\u9244",
+            "\u9244\u9244",
+            "\u9244\u9244\u9244",
+            "\uFFFF",
+            "\uFFFF\uFFFF",
+            "\uFFFF\uFFFF\uFFFF",
             "",
             string.Empty,
             "a",