Irregexp reentrancy (crrev.com/c/3162604) introduced a bug for global
regexp execution in which each iteration would use a new stack region
(i.e. we forgot to pop the regexp stack pointer when starting a new
iteration).

This CL fixes that by popping the stack pointer on the loop backedge.

At a high level:

- Initialize the backtrack_stackpointer earlier and avoid clobbering
  it by setup code.
- Pop it on the loop backedge.
- Slightly refactor Push/Pop operations to avoid unneeded memory
  accesses.

Bug: v8:11382
Change-Id: Ibad6235767e110089a2b346034f923590b286a05
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3194251Reviewed-by: Patrick Thier <pthier@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/main@{#77158}