[NFC][SimplifyCFG] Hoist 'original' DomTree verification from simplifyOnce() into run()

This is NFC since SimplifyCFG still currently defaults to not preserving DomTree.

SimplifyCFGOpt::simplifyOnce() is only be called from SimplifyCFGOpt::run(),
and can not be called externally, since SimplifyCFGOpt is defined in .cpp
This avoids some needless verifications, and is thus a bit faster
without sacrificing precision.