On ia32, the upper "half stack slot" must be located above the lower
half stack slot (in absolute address), hence the index is
"2 * index - 1" instead of "2 * index + 1". Note that the index
describes the negative offset from the stack pointer.

R=titzer@chromium.org

Bug: v8:7579
Change-Id: If207af405b126ab30043432d7934273e6e2a5330
Reviewed-on: https://chromium-review.googlesource.com/973301
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Ben Titzer <titzer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52116}