This CL introduces a new gn argument: v8_enable_pointer_compression which is
false by default. All the changes done in this CL are made under this flag.

Upper half-word of a Smi word must be properly sign-extended according to the
sign of the lower-half containing the actual Smi value.

Bug: v8:7703
Cq-Include-Trybots: luci.chromium.try:linux_chromium_rel_ng
Change-Id: I2b52ab49cd18c7c613130705de445fef44c30ac5
Reviewed-on: https://chromium-review.googlesource.com/1061175Reviewed-by: Yang Guo <yangguo@chromium.org>
Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Igor Sheludko <ishell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53519}