These indices were a mix of signed and unsigned.
Modulus on the signed integers was producing FXC warnings about performance.
Change-Id: Ib82f4296199a09d2f03be8b06314feefce0022e2
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/56765
Auto-Submit: Ben Clayton <bclayton@google.com>
Reviewed-by: David Neto <dneto@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>