When spilling a value to the stack, make sure to fill it as the same
type later. Otherwise, we might load garbage from the stack and violate
the assumption that the upper 32 bits of a 64 bit register are zero if
it currently holds a 32 bit value.

R=titzer@chromium.org

Bug: v8:7353, v8:6600
Change-Id: I7f2b1b31b7f3c13aa152c682cb59400fb5a3ebf0
Reviewed-on: https://chromium-review.googlesource.com/880682
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Ben Titzer <titzer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50797}