fixed SPIR-V compilation error with unmodified variables

Change-Id: I22981a60c21a2e16d41d69c53e099f221a202a58
Reviewed-on: https://skia-review.googlesource.com/11000
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
3 files changed