Fix inadvertently case sensitive Boyer-Moore (#39420)

* Unit test

* Fix

* Typo

* CR feedback

Author: danmoseley

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

@@ -1335,9 +1335,8 @@ protected void GenerateFindFirstChar()
                 }
 
                 // ch = runtext[runtextpos];
-                // if (ch == lastChar) goto partialMatch;
                 Rightchar();
-                if (_boyerMoorePrefix.CaseInsensitive && ParticipatesInCaseConversion(chLast))
+                if (_boyerMoorePrefix.CaseInsensitive)
                 {
                     CallToLower();
                 }
@@ -1349,6 +1348,7 @@ protected void GenerateFindFirstChar()
                     Ldloc(chLocal);
                     Ldc(chLast);
 
+                    // if (ch == lastChar) goto partialMatch;
                     BeqFar(lPartialMatch);
 
                     // ch -= lowAscii;

src/libraries/System.Text.RegularExpressions/tests/Regex.Match.Tests.cs

@@ -15,6 +15,8 @@ public class RegexMatchTests
         public static IEnumerable<object[]> Match_Basic_TestData()
         {
             // pattern, input, options, beginning, length, expectedSuccess, expectedValue
+            yield return new object[] { @"H#", "#H#", RegexOptions.IgnoreCase, 0, 3, true, "H#" }; // https://github.com/dotnet/runtime/issues/39390
+            yield return new object[] { @"H#", "#H#", RegexOptions.None, 0, 3, true, "H#" };
 
             // Testing octal sequence matches: "\\060(\\061)?\\061"
             // Octal \061 is ASCII 49 ('1')