[MLIR] Add type checking capability to RegionBranchOpInterface

- Add function `verifyTypes` that Op's can call to do type checking verification
  along the control flow edges described the Op's RegionBranchOpInterface.
- We cannot rely on the verify methods on the OpInterface because the interface
  functions assume valid Ops, so they may crash if invoked on unverified Ops.
  (For example, scf.for getSuccessorRegions() calls getRegionIterArgs(), which
  dereferences getBody() block. If the scf.for is invalid with no body, this
  can lead to a segfault). `verifyTypes` can be called post op-verification to
  avoid this.

Differential Revision: https://reviews.llvm.org/D82829