[mlir] Add lowering to CFG for WhileOp

The lowering is a straightforward inlining of the "before" and "after" regions
connected by (conditional) branches. This plugs the WhileOp into the
progressive lowering scheme. Future commits may choose to target WhileOp
instead of CFG when lowering ForOp.

Differential Revision: https://reviews.llvm.org/D90603