Reduce register pressure (and therefore spills) across the loop
iterating over the LoadIC polymorphic feedback array by starting at
length - 1 and iterating down to 0.

Might give a tiny recency boost too.

Change-Id: I1295a8136212c339b9d3974e2d49b3ecfe1ce543
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1687545Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/master@{#62528}