The mid-tier register allocator did not handle block merges correctly
where a SIMD register was partially overlapping with a non-SIMD
register. This CL fixes that, and reorders the code to allow for early
exits.

R=mslekova@chromium.org

Bug: chromium:1282224, v8:12330
Change-Id: I2e9275d5c1aaa764ecb63fbf8fa197b68d6b6c3c
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3358294Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Reviewed-by: Maya Lekova <mslekova@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/main@{#78482}