Irregexp code spills and restores ebx at boundaries, and clobbers it
inbetween. Root-relative loads are disabled in this region.

Bug: v8:6666
Change-Id: I9c68bc8ef88d324da609996a8457ac9e30a9b6fc
Reviewed-on: https://chromium-review.googlesource.com/c/1280762Reviewed-by: Sigurd Schneider <sigurds@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56639}