[maglev] Use free list for iterating used registers

Don't rely on register_values[index] == nullptr for checking if a
register is free, but instead re-use the free register list, and iterate
the allocatable_register & ~free_register list when iterating used
registers.

This also changes the indexing of register_values to be by register
code, not allocatable register index. The register state stored on the
InterpreterFrameState, however, stays compact (allocatable register
count). A new wrapper class + iterator keeps iteration over it and the
register_values array in sync.

Bug: v8:7700
Change-Id: I7815aa2d4a1f7b7ebafaaafe0727219adcc4dcfe
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3512792


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@{#79434}