Revert of [heap] Aggressive code flushing in GC stress mode. (patchset #1 id:1...

Revert of [heap] Aggressive code flushing in GC stress mode. (patchset #1 id:1 of https://codereview.chromium.org/1483993002/ )

Reason for revert:
This breaks (surprise) GC stress with custom snapshot, cannot reproduce, will need to investigate further.

https://build.chromium.org/p/client.v8/builders/V8%20Linux64%20GC%20Stress%20-%20custom%20snapshot/builds/2909/steps/Mjsunit/logs/deopt-inlined-from-call

Original issue's description:
> [heap] Aggressive code flushing in GC stress mode.
>
> This disregards the code age heuristic when deciding whether to flush
> code so that GC stress mode is more likely to flush out potential races
> between our various "invariants".
>
> R=hpayer@chromium.org
>
> Committed: https://crrev.com/28a3f23e50f1847dd1152e6bb4aa0579acbdaaab
> Cr-Commit-Position: refs/heads/master@{#32414}

TBR=hpayer@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true

Review URL: https://codereview.chromium.org/1485823002

Cr-Commit-Position: refs/heads/master@{#32416}

src/heap/objects-visiting-inl.h

@@ -627,6 +627,11 @@ bool StaticMarkingVisitor<StaticVisitor>::IsFlushable(Heap* heap,
     return false;
   }
 
+  // Check age of optimized code.
+  if (FLAG_age_code && !function->code()->IsOld()) {
+    return false;
+  }
+
   return IsFlushable(heap, shared_info);
 }
 
@@ -685,16 +690,6 @@ bool StaticMarkingVisitor<StaticVisitor>::IsFlushable(
     return false;
   }
 
-  // ----------------------------------------------------------------
-  // The above predicates up to this line are hard invariants, below
-  // this line are heuristics that should not affect correctness.
-  // ----------------------------------------------------------------
-
-  // In stress mode we are aggressive.
-  if (FLAG_stress_compaction) {
-    return true;
-  }
-
   // Check age of code. If code aging is disabled we never flush.
   if (!FLAG_age_code || !shared_info->code()->IsOld()) {
     return false;