If a fixed register is defined for an input, we did only spill the
sibling SIMD register if the other sibling was allocated. This is not
correct. If only the sibling is in use (e.g. s1 colliding with q0) we
also have to spill that sibling.

R=mslekova@chromium.org

Bug: chromium:1283042, v8:12330
Change-Id: I6a22eaf461774a0b4603ec3ff17062134a528161
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3359615Reviewed-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@{#78483}