Support indexing by loop variables in SkVMGenerator

Bug: skia:11096
Change-Id: I25a91bacf1c3455ac67422fb0e59b9b152c2054a
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/354667
Commit-Queue: Brian Osman <brianosman@google.com>
Reviewed-by: Mike Klein <mtklein@google.com>
9 files changed