[NewPM] Fix MergeFunctions scheduling

[NewPM] Fix MergeFunctions scheduling

MergeFunctions (as well as HotColdSplitting an IROutliner) are
incorrectly scheduled under the new pass manager. The code makes
it look like they run towards the end of the module optimization
pipeline (as they should), while in reality the run at the start.
This is because the OptimizePM populated around them is only
scheduled later.

I'm fixing this by moving these three passes until after OptimizePM
to avoid splitting the function pass pipeline. It doesn't seem
important to me that some of the function passes run after these
late module passes.

Differential Revision: https://reviews.llvm.org/D115098