[IROutliner] Handling outlined code with no exit paths

As a result of adding multiblock outlining, it became possible to outline the entirety of basic block, and branches that only pointed to the basic blocks contained in the outlined section. This means that there are no exit paths, and no return statement. There was a previous assertion from the older version of the outliner that explicitly made sure there was a return statement. This removes that assertion.

Reviewers: paquette

Differential Revision: https://reviews.llvm.org/D120868