On x64, the upper 32 bit of a register holding a 32-bit value are
always zero. Add several assertions to check that. This is particularly
important for memory accesses, where the whole 64-bit register is used
as offset for the memory access.

R=titzer@chromium.org

Bug: v8:6600
Change-Id: Ifd3c6e90bb0056dbc2b8c66de919f35c7787965b
Reviewed-on: https://chromium-review.googlesource.com/963321
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Ben Titzer <titzer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51947}