When generating a 64bit memory operation on ia32, we need to emit two
operations, one at {offset+4}, one at {offset}. The computation
{offset+4} can overflow, which is ok because
1) it won't be used for code generation later, and
2) the generated code will not be reached because the memory access is
   always out of bounds anyway.

R=ahaas@chromium.org

Bug: v8:7499, v8:6600
Change-Id: Ia4660688c3291700c48efc201d15fc370b4dd854
Reviewed-on: https://chromium-review.googlesource.com/939389Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51604}