[MLIR] Add argument related API to Region

- Arguments of the first block of a region are considered region arguments.
- Add API on Region class to deal with these arguments directly instead of
  using the front() block.
- Changed several instances of existing code that can use this API
- Fixes https://bugs.llvm.org/show_bug.cgi?id=46535

Differential Revision: https://reviews.llvm.org/D83599