The mips64 implementation always ended up in the slowpath due to some
loads that were the wrong width, so that is also fixed here.

BUG=v8:5974

Change-Id: Ie448a1fab5b7fca87597c5a1bf75443864e30c28
Reviewed-on: https://chromium-review.googlesource.com/443247
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#43222}