Implemented in both the reader and writer with E2E tests. Other
backends detect compound assignment and ICE for now.
Bug: tint:1325
Change-Id: Ie3f51e03627a38b12bd1513c4bcf1bebb3282863
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/74363
Reviewed-by: Ben Clayton <bclayton@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>
James Price