The existing implementation needed uses of the outputs of an
AtomicExchange to allocate registers for the result value. However,
these uses are not guaranteed to exist. With this CL temp registers
get allocated if the uses don't exist.

R=gdeepti@chromium.org

Bug: chromium:1077130
Change-Id: I058ee53b87c6e995c9f490f3aebbfdba69934f3c
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2179503Reviewed-by: Deepti Gandluri <gdeepti@chromium.org>
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67556}