Restructure code to make slow path of allocation more obvious.

Bug: v8:10315
Change-Id: Ic3e3b866b144b6f2877acac4accf87377f757172
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2276273
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68651}