The allocatable registers have holes, so not all FP registers are one
half of a valid SIMD register. Thus check if {GetAliases} returned an
allocatable SIMD register before looking up if that register is being
used. Otherwise we run into a DCHECK because {simd_reg} is invalid.

The bug was only introduced recently: https://crrev.com/c/3404780

R=thibaudm@chromium.org

Bug: chromium:1290079, v8:12330
Change-Id: I99df1645cfeec375daec82dbf41c110b5474339c
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3412075Reviewed-by: Thibaud Michaud <thibaudm@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/main@{#78742}