[DomTree] verifyDomTree to unconditionally perform DT verification

I folded the check for the flag -verify-dom-info into the only caller
where I think it is supposed to be checked: verifyAnalysis.  (The idea
of the flag is to enable this expensive verification in
verifyPreservedAnalysis.)

I'm assuming that when manually scheduling the verification pass
with -passes=verify<domtree>, we do want to perform the verification.

llvm-svn: 236575