[StructurizeCFG][NewPM] Port -structurizecfg to NPM

[StructurizeCFG][NewPM] Port -structurizecfg to NPM

This doesn't support -structurizecfg-skip-uniform-regions since that
would require porting LegacyDivergenceAnalysis.

The NPM doesn't support adding a non-analysis pass as a dependency of
another, so I had to add -lowerswitch to some tests or pin them to the
legacy PM.

This is the only RegionPass in tree, so I simply copied the logic for
finding all Regions from the legacy PM's RGManager into
StructurizeCFG::run().

Reviewed By: arsenm

Differential Revision: https://reviews.llvm.org/D89026