[IROutliner] Disallow outlining calls that return twice.

Functions that return twice can cause the IR Outliner to miscompile the given program. These function rely on information about the stack to be the same, and this may not necessarily be the case if called from an outlined function. So, we simply call these instructions illegal for the outliner to remove.

Reviewers: paquette

Differential Revision: https://reviews.llvm.org/D110007