If there is more than one value in the merge region, a stack-to-stack
move can overwrite the source of a stack-to-register move. To avoid
this, spill all registers.

R=clemensb@chromium.org

Bug: chromium:1299183
Change-Id: I10495434d0a18c9072ee3882e00a687edd8c592a
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3523044Reviewed-by: Clemens Backes <clemensb@chromium.org>
Commit-Queue: Thibaud Michaud <thibaudm@chromium.org>
Cr-Commit-Position: refs/heads/main@{#79584}