This also adds checks that ebx contains the root pointer during indirect load.
And we work around a few spots where we create an ebx Register but do not
actually reference it (e.g. when emitting xmm3, which has the same code as
ebx).

Bug: v8:6666
Change-Id: I7ec9e644c2e9c59d6395a71c6c5f479fac711d8d
Reviewed-on: https://chromium-review.googlesource.com/1231093
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Sigurd Schneider <sigurds@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56037}