Fix flaky failure in mjsunit/regress/regress-unlink-closures-on-deopt

This is short-term fix for flaky failures on GC fuzzer. The closures
g1 and g2 have the same SFI and pending optimzed table treats them as
a single entry. This cl, adds %PrepareFunctionForOptimize after one of
them is optimized.

Bug: v8:9556
Change-Id: I1fd72da1baa5de2f7650e080f9b6d04b69dd6a16
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1719188Reviewed-by: Georg Neis <neis@chromium.org>
Commit-Queue: Mythri Alle <mythria@chromium.org>
Cr-Commit-Position: refs/heads/master@{#62934}

test/mjsunit/regress/regress-unlink-closures-on-deopt.js

@@ -14,7 +14,6 @@ function foo() {
 let g1 = foo();
 let g2 = foo();
 %PrepareFunctionForOptimization(g1);
-%PrepareFunctionForOptimization(g2);
 
 g1({ f : 1});
 g1({ f : 2});
@@ -22,9 +21,10 @@ g2({ f : 2});
 g2({ f : 2});
 
 %OptimizeFunctionOnNextCall(g1);
-%OptimizeFunctionOnNextCall(g2);
-
 g1({ f : 1});
+
+%PrepareFunctionForOptimization(g2);
+%OptimizeFunctionOnNextCall(g2);
 g2({ f : 2});
 g1({});