We have a scratch available, so we can use it instead of asking for a
temporary register from the register allocator. We can also relax the
unique register requirement, since we are careful not to overwrite src0
or src1 in the codegen.

Bug: chromium:1204071
Change-Id: Ia7775167e323b3bca80f63304687cdbd425af0e5
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2873227Reviewed-by: Bill Budge <bbudge@chromium.org>
Commit-Queue: Zhi An Ng <zhin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#74392}