If we need a byte register, but {src} is none, we should definitely use
another register.

R=ahaas@chromium.org

Bug: chromium:1048241
Fixed: chromium:1048241
Change-Id: I3396826986e1823250ad6855b84f4b05faaf3b90
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2036073Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#66095}