- Introduce new operator variants for signed right-shifts with the
  additional information that they always shift out zeros.
- Use these new operators for Smi untagging.
- Merge left-shifts with a preceding Smi-untagging shift.
- Optimize comparisons of Smi-untagging shifts to operate on the
  unshifted word.
- Optimize 64bit comparisons of values expanded from 32bit to use
  a 32bit comparison instead.
- Change CodeStubAssembler::UntagSmi to first sign-extend and then
  right-shift to enable better address computations for Smi indices.

Bug: v8:9962
Change-Id: If91300f365e8f01457aebf0bd43bdf88b305c460
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2135734
Commit-Queue: Tobias Tebbi <tebbi@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67378}