Skip no-op stores when enforcing flag implications

The d8 shell modifies compiler flags in PrepareStressRun after isolate
was already set up and has run some JS code. Updating these flags
forces recomputation of implications for all flags.

This causes no-op stores to some unrelated flags that are accessed
from background threads leading to benign data races.

Bug: v8:10315
Change-Id: I568445d4382ae392970deccbf9588c98e46a4a4e
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2390140
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69672}

src/flags/flag-definitions.h

@@ -60,13 +60,13 @@
 // We produce the code to set flags when it is implied by another flag.
 #elif defined(FLAG_MODE_DEFINE_IMPLICATIONS)
 #define DEFINE_VALUE_IMPLICATION(whenflag, thenflag, value) \
-  if (FLAG_##whenflag) FLAG_##thenflag = value;
+  if (FLAG_##whenflag && FLAG_##thenflag != (value)) FLAG_##thenflag = (value);
 
 #define DEFINE_GENERIC_IMPLICATION(whenflag, statement) \
   if (FLAG_##whenflag) statement;
 
 #define DEFINE_NEG_VALUE_IMPLICATION(whenflag, thenflag, value) \
-  if (!FLAG_##whenflag) FLAG_##thenflag = value;
+  if (!FLAG_##whenflag && FLAG_##thenflag != (value)) FLAG_##thenflag = (value);
 
 // We apply a generic macro to the flags.
 #elif defined(FLAG_MODE_APPLY)
@@ -1078,8 +1078,8 @@ DEFINE_INT(stress_marking, 0,
 DEFINE_INT(stress_scavenge, 0,
            "force scavenge at random points between 0 and X (inclusive) "
            "percent of the new space capacity")
-DEFINE_IMPLICATION(fuzzer_gc_analysis, stress_marking)
-DEFINE_IMPLICATION(fuzzer_gc_analysis, stress_scavenge)
+DEFINE_VALUE_IMPLICATION(fuzzer_gc_analysis, stress_marking, 99)
+DEFINE_VALUE_IMPLICATION(fuzzer_gc_analysis, stress_scavenge, 99)
 DEFINE_BOOL(
     reclaim_unmodified_wrappers, true,
     "reclaim otherwise unreachable unmodified wrapper objects when possible")