SkVMCodeGenerator was lacking support for the comma operator entirely;
this has now been implemented.
Change-Id: I9350f54e6ee52764c620116e6dbfe4ca3e9cd47e
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/363096
Auto-Submit: John Stiles <johnstiles@google.com>
Reviewed-by: Mike Klein <mtklein@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>
