Introduce Liveness and Uses into existing scheduler

Liveness tracks all the live instructions in the instruction stream.
Uses maps this value to instructions that use it.

Uses is overkill for the current schedule, but will be needed for
spilling.

Change-Id: Id20b7b7a90901e156d323bb612c5908f91405967
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/277744
Commit-Queue: Herb Derby <herb@google.com>
Reviewed-by: Mike Klein <mtklein@google.com>
3 files changed