[MLIR][GPU] Add canonicalization patterns for folding simple gpu.wait ops.

* Fold away redundant %t = gpu.wait async + gpu.wait [%t] pairs.

* Fold away %t = gpu.wait async ... ops when %t has no uses.

* Fold away gpu.wait [] ops.

* In case of %t1 = gpu.wait async [%t0], replace all uses of %t1
  with %t0.

Differential Revision: https://reviews.llvm.org/D121878