It did not handle the case where dst == src. We switch the registers
used around to write to scratch first and ensure we don't overwrite dst.

Bug: chromium:1187831
Change-Id: Idf447aa1a3eff3920f2dfa3e0ec11efae37778cd
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2762425Reviewed-by: Deepti Gandluri <gdeepti@chromium.org>
Commit-Queue: Zhi An Ng <zhin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#73521}