When checking whether to merge a region with its surrounding regions in
{InsertIntoWritableRegions}, we did not check first whether the
determined {insert_pos} is within the vector. We were thus accessing
(reading) after the end of the vector.

The bug only happened on MSVC builds, suggesting that clang
deterministically read a value which is never equal to the end of the
new region, whereas for MSVC it sometimes happened that we read exactly
the {region.end()} value, and we tried to merge regions.

R=jkummerow@chromium.org

Bug: v8:12643
Change-Id: If30d910ed6e996f7b0e1d8c5b439c3d842a498f6
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3487988Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/main@{#79283}