[mlir][vector] Refactor TransferReadToVectorLoadLowering

* TransferReadToVectorLoadLowering no longer generates memref.load ops.
* Add new pattern VectorLoadToMemrefLoadLowering that lowers scalar vector.loads to memref.loads.
* Add vector::BroadcastOp canonicalization pattern that folds broadcast chains.

Differential Revision: https://reviews.llvm.org/D106117