fp_fixed1 and fp_fixed2 are used by the S8x16Shuffle operation. They
need to be allocatable, so that they can be correctly marked as fixed
and spilled as required. The previous value of fp_fixed2, d29, is not in
the list of allocatable double registers, and not marked as fixed
appropriately.

One fix could be to extend the list of allocatable double registers, but
there is a comment there saying that the list is kept even-length to
make stack alignment easier. So rather than messing with that, we
instead change what fp_fixed1 and fp_fixed2 is, since S8x16Shuffle is
the only user, this is a simpler change.

Bug: chromium:1070078
Change-Id: Id7de9b256bad2cfb11b0f06b66eb80a48ff7827c
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2161565Reviewed-by: Thibaud Michaud <thibaudm@chromium.org>
Reviewed-by: Deepti Gandluri <gdeepti@chromium.org>
Commit-Queue: Zhi An Ng <zhin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67372}