[IRSim][IROutliner] Add support for outlining PHINodes with the rest of the region.

We use the same similarity scheme we used for branch instructions for phi nodes, and allow them to be outlined. There is not a lot of special handling needed for these phi nodes when outlining, as they simply act as outputs. The code extractor does not currently allow for non entry blocks within the extracted region to have predecessors, so there are not conflicts to handle with respect to predecessors no longer contained in the function.

Recommit of 515eec35

Reviewers: paquette

Differential Revision: https://reviews.llvm.org/D106997