Merging register values can encounter constants, which are loadable but
don't have spill slots. Add support for these (in practice this is the
same behaviour, we're just fixing a DCHECK).

Bug: v8:7700
Change-Id: I9ab8ba1fc3a3a64fe16668bb317ad02f878f5849
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3749579
Auto-Submit: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Commit-Queue: Toon Verwaest <verwaest@chromium.org>
Cr-Commit-Position: refs/heads/main@{#81611}