The kContextRegister can alias allocated registers - when setting it,
take care not to unintentionally clobber.

Bug: v8:7700
Change-Id: I0635d334fb14fa15540582a4873d4186fffa2199
Fixed: chromium:1363450
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3897634
Auto-Submit: Jakob Linke <jgruber@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/main@{#83212}