The variable was incorrectly set to be a `vec3<f32>` instead of a
`vec3<u32>`.
Bug: tint:24
Change-Id: I84262a185925c50adf3a149f5f93f45124499596
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/18400
Reviewed-by: David Neto <dneto@google.com>