Add tighter bound to range check for matching Regex char classes

src/libraries/System.Text.RegularExpressions/gen/RegexGenerator.Emitter.cs

@@ -4006,8 +4006,8 @@ private static string MatchCharacterClass(bool hasTextInfo, RegexOptions options
                 // character class were [A-Za-z0-9], so since the ch is now known to be >= 128, we
                 // can just fail the comparison.
                 return negate ?
-                    $"((ch = {chExpr}) >= 128 || ({Literal(bitVectorString)}[ch >> 4] & (1 << (ch & 0xF))) == 0)" :
-                    $"((ch = {chExpr}) < 128 && ({Literal(bitVectorString)}[ch >> 4] & (1 << (ch & 0xF))) != 0)";
+                    $"((ch = {chExpr}) >= {Literal((char)analysis.UpperBoundExclusiveIfContainsOnlyAscii)} || ({Literal(bitVectorString)}[ch >> 4] & (1 << (ch & 0xF))) == 0)" :
+                    $"((ch = {chExpr}) < {Literal((char)analysis.UpperBoundExclusiveIfContainsOnlyAscii)} && ({Literal(bitVectorString)}[ch >> 4] & (1 << (ch & 0xF))) != 0)";
             }
 
             if (analysis.AllNonAsciiContained)

src/libraries/System.Text.RegularExpressions/src/System/Text/RegularExpressions/RegexCharClass.cs

@@ -921,6 +921,8 @@ internal struct CharClassAnalysisResults
             public bool AllAsciiContained;
             /// <summary>true if we know for sure that all non-ASCII values are in the set; otherwise, false.</summary>
             public bool AllNonAsciiContained;
+            /// <summary>The exclusive upper bound. Only valid if <see cref="ContainsOnlyAscii"/> is true.</summary>
+            public int UpperBoundExclusiveIfContainsOnlyAscii;
         }
 
         /// <summary>Analyzes the set to determine some basic properties that can be used to optimize usage.</summary>
@@ -962,6 +964,7 @@ internal static CharClassAnalysisResults Analyze(string set)
                 AllAsciiContained = false,
                 ContainsOnlyAscii = set[set.Length - 1] <= 128,
                 ContainsNoAscii = set[SetStartIndex] >= 128,
+                UpperBoundExclusiveIfContainsOnlyAscii = set[set.Length - 1],
             };
         }
 

src/libraries/System.Text.RegularExpressions/src/System/Text/RegularExpressions/RegexCompiler.cs

@@ -5021,7 +5021,7 @@ void EmitCharInClass()
 
             // ch < 128 ? (bitVectorString[ch >> 4] & (1 << (ch & 0xF))) != 0 :
             Ldloc(tempLocal);
-            Ldc(128);
+            Ldc(analysis.ContainsOnlyAscii ? analysis.UpperBoundExclusiveIfContainsOnlyAscii : 128);
             Bge(comparisonLabel);
             Ldstr(bitVectorString);
             Ldloc(tempLocal);