[vm/test] Avoid temporary changes to disassemble_optimized under TSAN.

Cf. d6e0590

TEST=tsan
Change-Id: I95c7cd9117b52577a846894d92d17dfe6516b862
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/417481
Reviewed-by: Alexander Aprelev <[email protected]>
Commit-Queue: Alexander Aprelev <[email protected]>

Author: rmacnak-google

runtime/vm/compiler/backend/il_test.cc

@@ -851,7 +851,7 @@ ISOLATE_UNIT_TEST_CASE(IRTest_CachableIdempotentCall) {
   }
 
   {
-#if !defined(PRODUCT)
+#if !defined(PRODUCT) && !defined(USING_THREAD_SANITIZER)
     SetFlagScope<bool> sfs(&FLAG_disassemble_optimized, true);
 #endif
     pipeline.CompileGraphAndAttachFunction();