This ensures that after the function-entry stack check, the instance
will still be available in a register. The cost is having to reload it
from the stack in the OOL code for the stack check, even though it is
not clear if that register will still be used.

This does not affect code size significantly (~0.25% reduction), but can
improve performance a little bit if there are memory accesses or other
instructions that require the instance right at the beginning of the
function.

R=thibaudm@chromium.org

Bug: v8:11336
Change-Id: Ib72db172813d55120f527b31014b69a734934ff3
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2743878Reviewed-by: Thibaud Michaud <thibaudm@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#73298}