Adds tests for scalable vectors in:
* sink-vector-broadcast.mlir
This test file excercises patterns grouped under
`populateSinkVectorBroadcastPatterns`, which includes:
* `ReorderElementwiseOpsOnBroadcast`,
* `ReorderCastOpsOnBroadcast`.
Right now there are only tests for the former. However, I've noticed
that "vector-reduce-to-contract.mlir" contains tests for the latter and
I've left a few TODOs to group these tests back together in one file.
Additionally, added some helpful `notifyMatchFailure` messages in
`ReorderElementwiseOpsOnBroadcast`.
efe3db2124