Handle the verifier at run() time in the PassManager instead of build time

Handle the verifier at run() time in the PassManager instead of build time

This simplifies a few parts of the pass manager, but in particular we don't add as many
verifierpass as there are passes in the pipeline, and we can now enable/disable the
verifier after the fact on an already built PassManager.

Reviewed By: rriddle

Differential Revision: https://reviews.llvm.org/D90669