Since const variables are  immutable, ignore SetMaybeAssigned for them.

Bug: chromium:999450, chromium:1000170, v8:8510
Change-Id: Idc1b71677b3d03bb63cc025017c119710b8f392d
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1782170
Commit-Queue: Dan Elphick <delphick@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Cr-Commit-Position: refs/heads/master@{#63579}