[mlir] Added new RegionBranchTerminatorOpInterface and adapted uses of hasTrait<ReturnLike>.

This CL adds a new RegionBranchTerminatorOpInterface to query information about operands that can be
passed to successor regions. Similar to the BranchOpInterface, it allows to freely define the
involved operands. However, in contrast to the BranchOpInterface, it expects an additional region
number to distinguish between various use cases which might require different operands passed to
different regions.

Moreover, we added new utility functions (namely getMutableRegionBranchSuccessorOperands and
getRegionBranchSuccessorOperands) to query (mutable) operand ranges for operations equiped with the
ReturnLike trait and/or implementing the newly added interface.  This simplifies reasoning about
terminators in the scope of the nested regions.

We also adjusted the SCF.ConditionOp to benefit from the newly added capabilities.

Differential Revision: https://reviews.llvm.org/D105018