Fix compilation failure from regex source generator with some lazy loops (#118132)

Because of some possible complicated control flow, the compiler thinks these variables are not deterministically initialized.

Author: stephentoub

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

@@ -3852,7 +3852,8 @@ void EmitLazy(RegexNode node)
                 {
                     startingPos = ReserveName("lazyloop_starting_pos");
                     sawEmpty = ReserveName("lazyloop_empty_seen");
-                    writer.WriteLine($"int {startingPos} = pos, {sawEmpty} = 0; // the lazy loop may match empty iterations");
+                    additionalDeclarations.Add($"int {startingPos} = 0, {sawEmpty} = 0;");
+                    writer.WriteLine($"{startingPos} = {sawEmpty} = 0; // the lazy loop may match empty iterations");
                 }
 
                 // If the min count is 0, start out by jumping right to what's after the loop.  Backtracking

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

@@ -280,6 +280,14 @@ public static IEnumerable<object[]> Match_MemberData()
             yield return (@"(abcd*?)+e", "abcde", RegexOptions.None, 0, 5, true, "abcde");
             yield return (@"(abcd*)+?e", "abcde", RegexOptions.None, 0, 5, true, "abcde");
             yield return (@"(abcd*?)+?e", "abcde", RegexOptions.None, 0, 5, true, "abcde");
+            yield return (@"(?:m(?:((e)?)??)|a)\b", "you m you", RegexOptions.None, 0, 9, true, "m");
+            yield return (@"(?:m(?:((e)?)??)|a)\b", "you me you", RegexOptions.None, 0, 10, true, "me");
+            yield return (@"(?:m(?:((e)?)??)|a)\b", "you a you", RegexOptions.None, 0, 9, true, "a");
+            yield return (@"(?:m(?:((e)?)??)|a)\b", "you and you", RegexOptions.None, 0, 11, false, "");
+            yield return (@"(?:m(?:|(e)?)|a)\b", "you m you", RegexOptions.None, 0, 9, true, "m");
+            yield return (@"(?:m(?:|(e)?)|a)\b", "you me you", RegexOptions.None, 0, 10, true, "me");
+            yield return (@"(?:m(?:|(e)?)|a)\b", "you a you", RegexOptions.None, 0, 9, true, "a");
+            yield return (@"(?:m(?:|(e)?)|a)\b", "you and you", RegexOptions.None, 0, 11, false, "");
 
             // Testing selected FindOptimizations finds the right prefix
             yield return (@"(^|a+)bc", " aabc", RegexOptions.None, 0, 5, true, "aabc");