In essence, it simplifies the pattern (in x64):
  movl register, ___
  movlsxlq register, register
into:
  movlsxlq register, ___

This makes the code smaller and run faster, without compromising.
We can do something similar for Arm64 too.

The cases for Pointer and Any seem to be trickier but there seems to be
room to improve as well.

Cq-Include-Trybots: luci.v8.try:v8_linux64_pointer_compression_rel_ng
Cq-Include-Trybots: luci.v8.try:v8_linux64_arm64_pointer_compression_rel_ng
Bug: v8:7703
Change-Id: I583bdfafdae9330be0a08ad1dd4c196e7de2f0d5
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1697246Reviewed-by: Sigurd Schneider <sigurds@chromium.org>
Commit-Queue: Santiago Aboy Solanes <solanes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#62676}