Extract i8x16.popcnt implementation into a macro-assembler function, and
reuse it in Liftoff.

We need an additional temporary XMMRegister whose lifetimes overlap with
dst and src, so make sure to pin those 2 when getting an unused
XMMRegister.

Bug: v8:11002
Change-Id: I13400b139add6f12316b3f398a796e6bf5a1ea7f
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2676921
Commit-Queue: Zhi An Ng <zhin@chromium.org>
Reviewed-by: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Deepti Gandluri <gdeepti@chromium.org>
Cr-Commit-Position: refs/heads/master@{#72615}