This relands commit 1a38573f.

The original change used a sequence of instruction in the test that
could not be scalar lowered properly.

Original change's description:
> [arm] Change fp_fixed registers to be allocatable registers
>
> 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/+/2161565
> Reviewed-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}

Bug: chromium:1070078
Change-Id: I02bb4b3ad03817318cbd0ee706c5ef4f20c845ba
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2165867Reviewed-by: Deepti Gandluri <gdeepti@chromium.org>
Commit-Queue: Zhi An Ng <zhin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67406}