[UnrollRuntime] Add DomTree verification under debug mode

NFC: This adds the dom tree verification under debug mode at a point
just before we start unrolling the loop. This allows us to verify dom
tree at a state where it is much smaller and before the unrolling
actually happens.
This also implies we do not need to run -verify-dom-info everytime to
see if the DT is in a valid state when we transform the loop for runtime
unrolling.

llvm-svn: 350334