The PrepareCallCFunction methods would take a scratch register as arguments when
they can simply use ip. The reason to do this, apart from simplifying the code,
is that TurboFan's code generator uses r9 as a scratch for this when it could
just as well use ip.

Bug: v8:6553
Change-Id: I0ed762d201f7ff8b8ed074da758227a8db95fca7
Reviewed-on: https://chromium-review.googlesource.com/558934Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Commit-Queue: Pierre Langlois <pierre.langlois@arm.com>
Cr-Commit-Position: refs/heads/master@{#46465}