We were only considering eliminating Stores of size Tagged. There doesn't
appear to be a reason why. This CL enables store store elimination for all
sizes.

For example, in pointer compression, it means that Compressed values can
be targeted. In 32 bit versions, it means that doubles can now be targeted.

This is safe under the assumption that every byte of a JS object is only
ever accessed through one offset. For instance, byte 15 of a given object
may be accessed using a two-byte read at offset 14, or a four-byte read at
offset 12, but never both in the same program.

Change-Id: I865d412ed5b4db53a0154cf4da6303c407fdbda7
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1746469Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Commit-Queue: Santiago Aboy Solanes <solanes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#63168}