Expand an optimization for short values to IgnoreCase in single-value…

… SearchValues<string>

src/libraries/System.Private.CoreLib/src/System/SearchValues/Strings/SingleStringSearchValuesThreeChars.cs

@@ -330,7 +330,8 @@ private bool TryMatch(ref char searchSpaceStart, int searchSpaceLength, ref char
                 // If the value is short (!TValueLength.AtLeast4Chars => 2 or 3 characters), the anchors already represent the whole value.
                 // With case-sensitive comparisons, we've therefore already confirmed the match, so we can skip doing so here.
                 // With case-insensitive comparisons, we applied a mask to the input, so while the anchors likely matched, we can't be sure.
-                if ((typeof(TCaseSensitivity) == typeof(CaseSensitive) && !TValueLength.AtLeast4Chars) ||
+                // If the value is composed of only ASCII letters, masking the input can't produce false positives, so we can also skip the verification step.
+                if (((typeof(TCaseSensitivity) == typeof(CaseSensitive) || typeof(TCaseSensitivity) == typeof(CaseInsensitiveAsciiLetters)) && !TValueLength.AtLeast4Chars) ||
                     TCaseSensitivity.Equals<TValueLength>(ref matchRef, _value))
                 {
                     offsetFromStart = (int)((nuint)Unsafe.ByteOffset(ref searchSpaceStart, ref matchRef) / 2);
@@ -362,7 +363,8 @@ private bool TryMatch(ref char searchSpaceStart, int searchSpaceLength, ref char
                 // If the value is short (!TValueLength.AtLeast4Chars => 2 or 3 characters), the anchors already represent the whole value.
                 // With case-sensitive comparisons, we've therefore already confirmed the match, so we can skip doing so here.
                 // With case-insensitive comparisons, we applied a mask to the input, so while the anchors likely matched, we can't be sure.
-                if ((typeof(TCaseSensitivity) == typeof(CaseSensitive) && !TValueLength.AtLeast4Chars) ||
+                // If the value is composed of only ASCII letters, masking the input can't produce false positives, so we can also skip the verification step.
+                if (((typeof(TCaseSensitivity) == typeof(CaseSensitive) || typeof(TCaseSensitivity) == typeof(CaseInsensitiveAsciiLetters)) && !TValueLength.AtLeast4Chars) ||
                     TCaseSensitivity.Equals<TValueLength>(ref matchRef, _value))
                 {
                     offsetFromStart = (int)((nuint)Unsafe.ByteOffset(ref searchSpaceStart, ref matchRef) / 2);