A SIMD register can "block" more than one FP register. In that case, no
virtual register will be assigned for one of the FP registers. This is
fine, we just need to detect and handle that case correctly.

R=thibaudm@chromium.org
CC=​leszeks@chromium.org

Bug: chromium:1271538, v8:12330
Change-Id: I7ec19229445c5ace0782f63945acb89322816540
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3293082Reviewed-by: Thibaud Michaud <thibaudm@chromium.org>
Reviewed-by: Maya Lekova <mslekova@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/main@{#78400}