movups is slower on older hardware (core2) than movaps, even if the
operand is aligned. (Not an issue on modern hardware).

Also move i8x16.splat(0x0F) to an external reference so we can load the
mask directly.

Bug: v8:11002
Change-Id: I0b01c27a142024d50b9faaa9e7bd6a1fe169e141
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2643242Reviewed-by: Deepti Gandluri <gdeepti@chromium.org>
Commit-Queue: Zhi An Ng <zhin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#72336}