fixed SPIR-V indexing a swizzle

Bug: skia:8543
Change-Id: I5d22de1b84fca9839215fff102dcac4b3a8063f1
Reviewed-on: https://skia-review.googlesource.com/c/181842
Reviewed-by: Chris Dalton <csmartdalton@google.com>
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
1 file changed