* Move fixed temporary allocation before arbitrary input allocation,
    so that fixed temporaries don't accidentally clobber the arbitrary
    input register. Now the input allocation will pick a different
    register.
  * For the above, make temporary allocation 'block' the register with a
    sentinel value, rather than marking it free, so that the subsequent
    input allocation knows not to use those registers (including
    spilling into them).
  * Similarly, move arbitrary input allocation after phi resolution when
    allocating control nodes, since phis may have fixed requirements.
  * Allow deopts to spill their inputs if they are not in registers and
    not yet loadable. This is done during the equivalent of input
    allocation for deopts.
  * Allow there to be multiple targets for a single source during gap
    move collection / cycle detection. There can still only be a single
    source per target, therefore there can only be one cycle for each
    connected component -- this is DCHECKed.
  * Make register validation more complete -- also walk the entire
    graph, and check whether value nodes' result register states match
    the current register allocator state.
  * Add much more printing to --trace-maglev-regalloc because these bugs
    ain't easy to debug.

Bug: v8:7700
Change-Id: Id98259c2920d772ce168bf27497162e78b136f9f
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3714235
Auto-Submit: Toon Verwaest <verwaest@chromium.org>
Commit-Queue: Toon Verwaest <verwaest@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Commit-Queue: Igor Sheludko <ishell@chromium.org>
Cr-Commit-Position: refs/heads/main@{#81252}