Reland "Support indexing by loop variables in SkVMGenerator"

This reverts commit b7e836cee99b981882c2523bfe293e95cff51942.

Change-Id: I3c39a928ba4a9a2863b616f2a500975294b03860
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/355980
Commit-Queue: Brian Osman <brianosman@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>
Auto-Submit: Brian Osman <brianosman@google.com>
Reviewed-by: John Stiles <johnstiles@google.com>
Reviewed-by: Mike Klein <mtklein@google.com>
9 files changed