This ensures that i32 values which are loaded into 64 bit registers
have the upper 32 bits cleared.

R=titzer@chromium.org

Bug: v8:6600
Change-Id: I5eb2b65ac079b5683c83d755b1aa4a626411d5d4
Reviewed-on: https://chromium-review.googlesource.com/888702Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50907}