Use a transform to convert these to the vector form for the MSL and
SPIR-V backends.
MSL only has the scalar form from version 2.0 onwards.
Fixed: tint:1123
Change-Id: I384abd9872d9eae52a10a37cbd6aa96004692e9c
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/67360
Kokoro: Kokoro <noreply+kokoro@google.com>
Commit-Queue: James Price <jrprice@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>
James Price