[RFC] Add scf.while to scf dialect

[aartbik]

@aartbik

Since the block-arg/yield takes the role of a phi-node

You can see how I implemented the conditional induction for sparse computations by yielding in every branch and merge point in MLIR Support for Sparse Tensors - #5 by aartbik.

Many thanks to @ftynse for adding the while in such a timely fashion!