We update RegisterMerge information for DoubleRegister, but don't
actually emit the gap moves for them. This required templatifying some
more code on the register type, and exposing a general LoadToRegister
for ValueNode.

Bug: v8:7700
Change-Id: I7122b5c562bab20d8f912936ff150d15b9cc033f
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3785003
Auto-Submit: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Victor Gomes <victorgomes@chromium.org>
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/main@{#81927}