[mlir][SCF] Add a ParallelCombiningOpInterface to decouple...

[mlir][SCF] Add a ParallelCombiningOpInterface to decouple scf::PerformConcurrently from its contained operations

This allows purging references of scf.ForeachThreadOp and scf.PerformConcurrentlyOp from
ParallelInsertSliceOp.
This will allowmoving the op closer to tensor::InsertSliceOp with which it should share much more
code.

In the future, the decoupling will also allow extending the type of ops that can be used in the
parallel combinator as well as semantics related to multiple concurrent inserts to the same
result.

Differential Revision: https://reviews.llvm.org/D128857