Replace complex `count_returns` method with a ProgramVisitor.
Replace complex `count_returns` method with a ProgramVisitor.

These ProgramVisitor classes do the exact same work as count_returns.
Hopefully they are easier for us to reason about, since we no longer
have one omni-method trying to implement three different counts at once.

count_returns still exists in this CL so we can SkASSERT that our
results are unchanged. It is removed entirely in the followup CL.

Change-Id: I17588bcec375a946da6cc6fdb5bab9a3d7d24904
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/313683
Commit-Queue: John Stiles <f4fcf42d3bb5924557f1eeb3be66747535e585da@google.com>
Auto-Submit: John Stiles <f4fcf42d3bb5924557f1eeb3be66747535e585da@google.com>
Reviewed-by: Ethan Nicholas <d2cfb8c2f603266f4dc3a6f09acbcc0089603b37@google.com>
Reviewed-by: Brian Osman <794c0b5534edf5601d88e1d41975d0262da12894@google.com>

google/skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,google/skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,google/skia,aosp-mirror/platform_external_skia,google/skia,google/skia,aosp-mirror/platform_external_skia,google/skia,google/skia,google/skia,google/skia,aosp-mirror/platform_external_skia,google/skia,aosp-mirror/platform_external_skia