[IROutliner] Accomodate blocks containing PHINodes with one entry outside the region...

[IROutliner] Accomodate blocks containing PHINodes with one entry outside the region and others inside the region.

When a PHINode has an incoming block from outside the region, it must be handled specially when assigning a global value number to each incoming value. A PHINode has multiple predecessors, and we must handle this case rather than only the single predecessor case.

Reviewer: paquette

Differential Revision: https://reviews.llvm.org/D124777