IsCompiledScope retains code to be safe against bytecode flushing, but
%PrepareFunctionForOptimization isn't currently initializing it with the
function's current compiled state. IOW, it's only retaining freshly
compiled code and is causing flakes for already-compiled functions.

Bug: v8:12697
Change-Id: Ie82a4adb8a136da708b3ae0ce27a42f5c277d324
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3668318Reviewed-by: Adam Klein <adamk@chromium.org>
Commit-Queue: Shu-yu Guo <syg@chromium.org>
Cr-Commit-Position: refs/heads/main@{#80756}