[MLIR] Allow Loop dialect IfOp and ForOp to define values

This patch implements the RFCs proposed here:
https://llvm.discourse.group/t/rfc-modify-ifop-in-loop-dialect-to-yield-values/463
https://llvm.discourse.group/t/rfc-adding-operands-and-results-to-loop-for/459/19.

It introduces the following changes:
- All Loop Ops region, except for ReduceOp, terminate with a YieldOp.
- YieldOp can have variadice operands that is used to return values out of IfOp and ForOp regions.
- Change IfOp and ForOp syntax and representation to define values.
- Add unit-tests and update .td documentation.
- YieldOp is a terminator to loop.for/if/parallel
- YieldOp custom parser and printer

Lowering is not supported at the moment, and will be in a follow-up PR.

Thanks.

Reviewed By: bondhugula, nicolasvasilache, rriddle

Differential Revision: https://reviews.llvm.org/D74174