Bring ia32 in line with the arm implemention. 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: Ia3799a42fe2b9890aa4076d2e09947380331348e
Reviewed-on: https://chromium-review.googlesource.com/c/1391758
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58501}