It was slightly inconsistent, the sse versions took int8_t, the avx
versions took uint8_t. Consolidate into uint8_t, that allows us to
remove the DCHECK inside of the assembler, and also convert callers to
use uint8_t.

Bug: v8:10933
Change-Id: I125f0d54533b6fde1362e63e96f50fcf2467cac5
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2443494Reviewed-by: Bill Budge <bbudge@chromium.org>
Commit-Queue: Zhi An Ng <zhin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70274}