The object itself is already decompressed, and we're simply re-decompressing by
nuking the upper bits through sign extension.

Additionally this CL changes the branchless decompression sequence on x64 to be
cmov-based since that's shorter and faster. It's still slower than branchful
though, so we likely won't use it.

Change-Id: Ie6f9d38fb390b7300a236bf85d0db58d1ee959b0
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1701842Reviewed-by: Sigurd Schneider <sigurds@chromium.org>
Commit-Queue: Toon Verwaest <verwaest@chromium.org>
Cr-Commit-Position: refs/heads/master@{#62708}