Bring mips in line with the ia32 and arm implementions. Instead of
computing the "half index" manually, pass the index and an enum
pointing to either half.

R=titzer@chromium.org

Bug: v8:6600
Change-Id: Id8812114df84fbddd4aa2570306526d291bface6
Reviewed-on: https://chromium-review.googlesource.com/c/1392188
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58514}