We should only check the "SIMD sibling" register if we are handling a
SIMD register. This avoids unneeded spills, and in this particular case
ran into a DCHECK because there are only 29 registers, but we tried
checking #29.

R=thibaudm@chromium.org

Bug: v8:12330, v8:1285007
Change-Id: Ife8b295ac958990611ca8816bbfbfb5124a4297d
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3372916Reviewed-by: Thibaud Michaud <thibaudm@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/main@{#78532}