[LegacyPM] Port example pass SimplifyCFG to new PM

This is part of effort in removing -enable-new-pm flag.
As a prat of this effort one of example passes SimplifyCFG must
be ported to new PM which will allow to remove the flag
calls from the tests that are using this pass.

Reviewed By: aeubanks

Differential Revision: https://reviews.llvm.org/D137103