Refactor the ExpandCompoundAssignment transform to handle these
statements, which delivers support for all of the non-WGSL backends.
Fixed: tint:1488
Change-Id: I96cdc31851c61f6d92d296447d0b0637907d5fe5
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/86004
Reviewed-by: Ben Clayton <bclayton@google.com>