PPC [simd]: cleanup the instruction selector

This CL includes the following changes:
- Avoid using `UniqueRegister` as much as possible
- Try to group opcodes under Binary or Unary when possible

Some codegen ops had to also be modified to avoid using `Temp`
registers.

Change-Id: Ib21ab7a47f600068c8453d48c3549e481a19c328
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3780496Reviewed-by: Junliang Yan <junyan@redhat.com>
Commit-Queue: Milad Farazmand <mfarazma@redhat.com>
Cr-Commit-Position: refs/heads/main@{#82011}