Fix merge return in the face of breaks (#2466)

Fixes #2453

* Enable addition of OpPhi instructions when the loop has multiple
predecessors of the merge due to a break
 * This can result in some values no longer dominating their uses
* Track return blocks in structured flow to produce OpPhis that have
multiple undef and non-undef arguments
* New tests to catch the bug
* When a block is predicated, mark the new body as a return if the old
block as already a return