Exception handlers were allowing register state to leak through, which
had knock-on effects of Phi allocation inserting gap moves in an illegal
location (specifically, at the end of the block, thinking that it's
allocating a control node since it's not allocating a body node).

Fix the register leak by clearing register state, and add some invariant
guards in the areas where the failure appeared.

Bug: v8:7700
Change-Id: I15c1fba1a250e295f0147a4e51a6c8c5481e8c7e
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3890989Reviewed-by: Jakob Linke <jgruber@chromium.org>
Commit-Queue: Jakob Linke <jgruber@chromium.org>
Auto-Submit: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/main@{#83129}