reframe liveness_analysis as eliminate_dead_code

Seems nicer to keep encapsulated in a program->program pass
so nothing upstream of it has to think about liveness.

I will be circling back around to profiling the cost of these
tempoaries, copies, etc.  I just want to start writing them as
if cost were no object first.

Change-Id: I1d1187b521fbe963e720e0d8de90316a549f7797
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/281182
Reviewed-by: Herb Derby <herb@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
3 files changed