[mlir] Add a simpler lowering pattern for WhileOp representing a do-while loop

When the "after" region of a WhileOp is merely forwarding its arguments back to
the "before" region, i.e. WhileOp is a canonical do-while loop, a simpler CFG
subgraph that omits the "after" region with its extra branch operation can be
produced. Loop rotation from general "while" to "if { do-while }" is left for a
future canonicalization pattern when it becomes necessary.

Differential Revision: https://reviews.llvm.org/D90604