Instead of spilling x0 and x1 to the stack, use temp registers which
should always be available in this situation. This avoids the spill and
load.
Also, switch from holding {start} and {end} in the register to holding
{start} and {count}. This make it easier to correctly initialize the
registers.

R=joey.gouly@arm.com

Bug: v8:9830
Change-Id: I5fc8b8603afaf12fad2ba66cd7a75555ef31bd4b
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1859618
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#64332}