Regex fuzzer: Add return statement inside a randomly-chosen function

Adds a return statement in the body of a randomly-chosen function.
The return value is a randomly-chosen identifier or literal from
the WGSL shader.

Fixes: tint:1115.

Change-Id: Icdc4ff669cda343244e158ce791b4085fd52f7b9
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/61781
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: Alastair Donaldson <afdx@google.com>
Commit-Queue: Alastair Donaldson <afdx@google.com>

fuzzers/tint_regex_fuzzer/fuzzer.cc

@@ -37,6 +37,7 @@ enum class MutationKind {
   kDuplicateInterval,
   kReplaceIdentifier,
   kReplaceLiteral,
+  kInsertReturnStatement,
   kNumMutationKinds
 };
 
@@ -95,6 +96,12 @@ extern "C" size_t LLVMFuzzerCustomMutator(uint8_t* data,
       }
       break;
 
+    case MutationKind::kInsertReturnStatement:
+      if (!InsertReturnStatement(wgsl_code, generator)) {
+        return 0;
+      }
+      break;
+
     default:
       assert(false && "Unreachable");
       return 0;