Loops will always execute a stack check in the header, so having the
instance cached in a register is handy. Instead of clearing it before
entering a loop, ensure that backward jumps to the loop header move the
instance into the right register.

R=thibaudm@chromium.org

Bug: v8:11336
Change-Id: I16cb13457438b7a1603182d56a3d2ea99d670911
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2743892Reviewed-by: Thibaud Michaud <thibaudm@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#73317}