[IROutliner] Avoid reusing PHINodes that have already been matched when...

[IROutliner] Avoid reusing PHINodes that have already been matched when merging outlined functions' phi node blocks

When there are two external phi nodes for two different outlined regions, when compressing the created phi nodes between the two regions, the matching for the second phi node in the second region matches the first phi node created for the first region rather than the second phi node created for the first region. This adds an extra output path where there should not be one.

The fix is the ignore phi nodes that have already been matched for each region.

Reviewer: paquette

Differential Revision: https://reviews.llvm.org/D121312