Handle the case when one or both of the output nodes of an I64Atomic op
are optimized, for code-gen instructions that use a set of fixed
registers, use temp registers to ensure the registers are not
clobbered.

BUG:v8:6532


Change-Id: I52763c48d615cdf3ae8d754402b11da2df31a4a1
Reviewed-on: https://chromium-review.googlesource.com/1195910Reviewed-by: Bill Budge <bbudge@chromium.org>
Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Deepti Gandluri <gdeepti@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56004}