Revert of removing equality / compute invariant loops from GrGeometryProcessors ...

Revert of removing equality / compute invariant loops from GrGeometryProcessors (patchset #2 id:20001 of https://codereview.chromium.org/1111603004/)

Reason for revert:
breaks gl programs

Original issue's description:
> removing equality / compute invariant loops from GrGeometryProcessors
>
> BUG=skia:
>
> Committed: https://skia.googlesource.com/skia/+/ef292a0901205b9785a30daae2c036aa34a970ca

TBR=bsalomon@google.com,joshualitt@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=skia:

Review URL: https://codereview.chromium.org/1110993002