perf tweaks
perf tweaks

  - cheap search for available registers before
    falling back on expensive scoring (big deal)
  - free dying registers proactively (medium)
  - free TMP registers manually (peanuts)

  This drops the SkVM_Overhead_VM bench from 49µs to
  38µs on my laptop.  It was 32µs before the refactor,
  so there may still be some potatoes to dig up here.

Change-Id: I6822613e9186278e80ec045e84517ac3a32d6f43
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/284832
Reviewed-by: Herb Derby <9e12a3d2bbf73546e47cc9958f6ba8f3215b7eaf@google.com>
Commit-Queue: Mike Klein <14574f09dfa9b4e14759b88c3426a495a0e627b0@google.com>

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