Move the helper class and some function definitions into
SharedTurboAssembler. We leave most of the other function definitions
inside of macro-assembler-x64, and will move them later.

Also move i16x8.ext_mul high as a check that this code movement works.

Bug: v8:11589
Change-Id: I8ec1fa24cb93b4c4c8bd936a9df06cbf5328374f
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2792080Reviewed-by: Bill Budge <bbudge@chromium.org>
Commit-Queue: Zhi An Ng <zhin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#73750}