simplify a confusing expression in SkSLSPIRVCodeGenerator

Bug: skia:
Change-Id: I3ce0c174677cf93903a5fb33287dfe1df3445837
Reviewed-on: https://skia-review.googlesource.com/18141
Reviewed-by: Greg Daniel <egdaniel@google.com>
Reviewed-by: Bruce Dawson <brucedawson@google.com>
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
1 file changed