The object is not always an array. The index can be applied to vectors
too.
Change-Id: Ifb63d1862090d28cb48d692870e9dd01ddbce5df
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/68841
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: James Price <jrprice@google.com>