Add a few missing char.IsXx helpers usage to RegexGenerator.Emitter.cs (#75101)

Just pretties up the generated code if these ranges are used.  With this, all of the char.IsXx helpers can be emitted.

Author: stephentoub

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

@@ -4404,9 +4404,13 @@ private static string MatchCharacterClass(RegexOptions options, string chExpr, s
                 negate ^= RegexCharClass.IsNegated(charClass);
                 return (lowInclusive, highInclusive) switch
                 {
+                    ('\0', '\u007F') => $"{(negate ? "!" : "")}char.IsAscii({chExpr})",
                     ('0', '9') => $"{(negate ? "!" : "")}char.IsAsciiDigit({chExpr})",
                     ('a', 'z') => $"{(negate ? "!" : "")}char.IsAsciiLetterLower({chExpr})",
                     ('A', 'Z') => $"{(negate ? "!" : "")}char.IsAsciiLetterUpper({chExpr})",
+                    ('\ud800', '\udfff') => $"{(negate ? "!" : "")}char.IsSurrogate({chExpr})",
+                    ('\ud800', '\udbff') => $"{(negate ? "!" : "")}char.IsHighSurrogate({chExpr})",
+                    ('\udc00', '\udfff') => $"{(negate ? "!" : "")}char.IsLowSurrogate({chExpr})",
                     _ when lowInclusive == highInclusive => $"({chExpr} {(negate ? "!=" : "==")} {Literal(lowInclusive)})",
                     _ => $"{(negate ? "!" : "")}char.IsBetween({chExpr}, {Literal(lowInclusive)}, {Literal(highInclusive)})",
                 };

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

@@ -85,7 +85,7 @@ internal abstract class RegexCompiler
         private static readonly MethodInfo s_arrayResize = typeof(Array).GetMethod("Resize")!.MakeGenericMethod(typeof(int));
         private static readonly MethodInfo s_mathMinIntInt = typeof(Math).GetMethod("Min", new Type[] { typeof(int), typeof(int) })!;
         // Note:
-        // IsAsciiLetterLower, IsAsciiLetterUpper, IsAsciiDigit, and IsBetween aren't used here, as the IL generated for those
+        // Single-range helpers like IsAsciiLetterLower, IsAsciiLetterUpper, IsAsciiDigit, and IsBetween aren't used here, as the IL generated for those
         // single-range checks is as cheap as the method call, and there's no readability issue as with the source generator.
 
         /// <summary>The ILGenerator currently in use.</summary>