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

Bug: v8:11002
Change-Id: I86b2f5322c799d44f584cac28c70e0e393bf114f
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2676280Reviewed-by: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Deepti Gandluri <gdeepti@chromium.org>
Commit-Queue: Zhi An Ng <zhin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#72565}