With this change, we use the same heuristics for picking free registers
and for picking a blocked register to spill. This is mostly cleanup to
make results less surprising.

Change-Id: I09d031c758ffbd02798d573bc9e68a474e916418
Reviewed-on: https://chromium-review.googlesource.com/c/1411607
Commit-Queue: Stephan Herhut <herhut@chromium.org>
Reviewed-by: Sigurd Schneider <sigurds@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58948}