Because of missing parentheses, the computation of the "half index" was
wrong, and always produced 0 or 1.
Also, for non-pairs, we were still passing kHighWord for the
RegPairHalf.

R=ahaas@chromium.org

Bug: v8:7422, v8:6600
Change-Id: If056aa8005d4b44e667b7d76b9be49ec0191d0eb
Reviewed-on: https://chromium-review.googlesource.com/908554Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51175}