Additional code gen improvements

- In Go, Improve code generation from several constructs to reduce code size and increase bounds reduction in the non-backtracking compiler
- In FindFirstChars, use IndexOf instead of Boyer-Moore if the found prefix is only one character in length.
- Hide some debug-only functions from code coverage

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

@@ -26,6 +26,7 @@
 //
 
 using System.Collections;
+using System.Diagnostics.CodeAnalysis;
 using System.Globalization;
 
 namespace System.Text.RegularExpressions
@@ -331,6 +332,7 @@ internal void Tidy(int textpos)
         }
 
 #if DEBUG
+        [ExcludeFromCodeCoverage]
         internal bool Debug => _regex != null && _regex.Debug;
 
         internal virtual void Dump()
@@ -372,6 +374,7 @@ internal MatchSparse(Regex regex, Hashtable caps, int capcount, string text, int
         public override GroupCollection Groups => _groupcoll ??= new GroupCollection(this, _caps);
 
 #if DEBUG
+        [ExcludeFromCodeCoverage]
         internal override void Dump()
         {
             if (_caps != null)

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

@@ -461,6 +461,7 @@ protected void InitializeReferences()
         /// <summary>
         /// True if the regex has debugging enabled
         /// </summary>
+        [ExcludeFromCodeCoverage]
         internal bool Debug => (roptions & RegexOptions.Debug) != 0;
 #endif
     }

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

@@ -12,6 +12,7 @@
 // need to be examined.
 
 using System.Diagnostics;
+using System.Diagnostics.CodeAnalysis;
 using System.Globalization;
 
 namespace System.Text.RegularExpressions
@@ -338,8 +339,10 @@ public int Scan(string text, int index, int beglimit, int endlimit)
         /// <summary>
         /// Used when dumping for debugging.
         /// </summary>
+        [ExcludeFromCodeCoverage]
         public override string ToString() => Pattern;
 
+        [ExcludeFromCodeCoverage]
         public string Dump(string indent)
         {
             StringBuilder sb = new StringBuilder();

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

@@ -4,6 +4,7 @@
 
 using System.Collections.Generic;
 using System.Diagnostics;
+using System.Diagnostics.CodeAnalysis;
 using System.Globalization;
 using System.Threading;
 
@@ -1374,6 +1375,7 @@ private static ReadOnlySpan<char> SetFromProperty(string capname, bool invert, s
         /// <summary>
         /// Produces a human-readable description for a set string.
         /// </summary>
+        [ExcludeFromCodeCoverage]
         public static string SetDescription(string set)
         {
             int setLength = set[SetLengthIndex];
@@ -1472,6 +1474,7 @@ public static string SetDescription(string set)
         /// <summary>
         /// Produces a human-readable description for a single character.
         /// </summary>
+        [ExcludeFromCodeCoverage]
         public static string CharDescription(char ch)
         {
             if (ch == '\\')
@@ -1507,6 +1510,7 @@ public static string CharDescription(char ch)
             return sb.ToString();
         }
 
+        [ExcludeFromCodeCoverage]
         private static string CategoryDescription(char ch)
         {
             if (ch == SpaceConst)

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

@@ -16,8 +16,8 @@
 // Strings and sets are indices into a string table.
 
 using System.Collections;
-using System.Collections.Generic;
 using System.Diagnostics;
+using System.Diagnostics.CodeAnalysis;
 
 namespace System.Text.RegularExpressions
 {
@@ -241,6 +241,7 @@ public static int OpcodeSize(int opcode)
             "Oneloopatomic", "Notoneloopatomic", "Setloopatomic"
         };
 
+        [ExcludeFromCodeCoverage]
         private static string OperatorDescription(int Opcode)
         {
             bool isCi = ((Opcode & Ci) != 0);
@@ -256,6 +257,7 @@ private static string OperatorDescription(int Opcode)
                 (isBack2 ? "-Back2" : "");
         }
 
+        [ExcludeFromCodeCoverage]
         public string OpcodeDescription(int offset)
         {
             StringBuilder sb = new StringBuilder();
@@ -367,6 +369,7 @@ public string OpcodeDescription(int offset)
             return sb.ToString();
         }
 
+        [ExcludeFromCodeCoverage]
         public void Dump()
         {
             int i;