The result of SmiUntag is a sign-extended word-size value.

Bug: v8:7703
Change-Id: I85dc87b541cab78286e47e2147c16c6a0939f509
Reviewed-on: https://chromium-review.googlesource.com/1073232Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Commit-Queue: Igor Sheludko <ishell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53389}