swift234

Reapply "Verifier: Cannonical SIL should have split critical edges for non cond_br
terminators"

This is an assumption that the SSAUpdater makes. Verify that we preserve this
property.

With changes to the test cases, SIL documentation and add a critical edge (non
cond_br only) splitting pass to the mandatory pipeline.

This reapplies commit 22775.

Swift SVN r22803