[turbofan] Fix some tests

- serializer-*:
In some stress configuration, the new map of x was GC'd at the beginning
of optimization, thus generating a soft-deopt for the store to x (thus
in turn skipping inlining of f).

- native-context-*:
In some stress configuration, f had its feedback flushed.

Bug: v8:10892
Change-Id: Icd9f9c0ba6feb938ae8c3b0031b02b766f2e3f91
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2404764
Commit-Queue: Georg Neis <neis@chromium.org>
Commit-Queue: Mythri Alle <mythria@chromium.org>
Reviewed-by: Mythri Alle <mythria@chromium.org>
Auto-Submit: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69837}

test/mjsunit/compiler/native-context-specialization-hole-check.js

@@ -38,7 +38,7 @@ function f() {
   arr[500] = 20;
   arr[10] = arr[50];
 }
-%EnsureFeedbackVectorForFunction(f);
+%PrepareFunctionForOptimization(f);
 
 function g() {
   f();

test/mjsunit/compiler/serializer-dead-after-jump.js

@@ -16,6 +16,7 @@ function main(b, ret) {
     return ret;
   } else {
     x.foo = 42;
+    out = x;  // Prevent x's new map from dying too early.
     return f(y);
   }
 }

test/mjsunit/compiler/serializer-dead-after-return.js

@@ -15,6 +15,7 @@ function main(b, ret) {
   if (b) return ret;
 
   x.foo = 42;
+  out = x;  // Prevent x's new map from dying too early.
   return f(y);
 }