[RewriteStatepointsForGC] Fix a latent bug in normalization for invoke statepoint [NFC]

Since we're restructuring the CFG, we also need to make sure to update the analsis passes. While I'm touching the code, I dedicided to restructure it a bit.  The code involved here was very confusing.  This change moves the normalization to essentially being a pre-pass before the main insertion work and updates a few comments to actually say what is happening and *why*.

The restructuring should be covered by existing tests.  I couldn't easily see how to create a test for the invalidation bug.  Suggestions welcome.

llvm-svn: 234769