This swaps out ebx in favor of esi. The root pointer value itself is
pushed and restored from the stack through pushad/popad.

Bug: v8:6666
Change-Id: I2a685f6659a5fbe2f7d91e431c5addff85664b5b
Reviewed-on: https://chromium-review.googlesource.com/1238653
Commit-Queue: Sigurd Schneider <sigurds@chromium.org>
Reviewed-by: Sigurd Schneider <sigurds@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56162}