Make sure that:

  * Temporaries are consistently free-but-blocked,
  * Blocked registers are ignored when processing free registers (where
    appropriate),
  * Fixed phis are processed before arbitrary register allocation,
  * Blocked state is set and cleared correctly
  * Opportunistic register moves on dropping don't block registers

Bug: v8:7700
Change-Id: I2bc8884f70d9e54ce6ee2fb5bb600b028a9502c3
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3732931
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Auto-Submit: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/main@{#81435}