DoubleRegister RegisterMerge could get assigned a Tagged representation
operand if the node in the merge was null.

Also, we had a wrong DCHECK when materialising these moves, as a result
of templatifying the gap move implementation.

Bug: v8:7700
Change-Id: I2a425a6bedf4c67e8acf6c30c877e055b6445b12
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3783994
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Auto-Submit: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Victor Gomes <victorgomes@chromium.org>
Cr-Commit-Position: refs/heads/main@{#82018}