According to the style guide, the implicit conversion of any number of
registers to a LiftoffRegList should not be there. This CL removes it,
and fixes two subideal call sites to use SpillRegister (receiving a
single register) instead of SpillOneRegister (receiving a register list
to choose from).

Plus some semantics-preserving rewrites.

R=jkummerow@chromium.org

Bug: chromium:1337221
Change-Id: Id22043ac1c185bc794dbde7baa4b1d5ab7cce56e
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3707286Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Commit-Queue: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/main@{#81250}