No functionality change, moved the i8x16.popcnt algorithm
into shared-macro-assembler.

Bug: v8:11589
Change-Id: I3dd9d01589bf0176df1e33433f4c3c0c717c253d
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3122572Reviewed-by: Adam Klein <adamk@chromium.org>
Commit-Queue: Zhi An Ng <zhin@chromium.org>
Cr-Commit-Position: refs/heads/main@{#76516}