[Dominators] Do not perform expensive checks by default. Fix PR33656.

Summary:
Some transforms assume that DT.verifyDomInfo() is not expensive and call it even when ENABLE_EXPENSIVE_CHECKS is not set.
This patch disables expensive Dominator Tree verification (reachability, parent property, sibling property) to fix
[[ https://bugs.llvm.org/show_bug.cgi?id=33656 | PR33656 ]].

Note that this is only a temporary fix.

Reviewers: dberlin, chapuni, kparzysz, grosser

Reviewed By: dberlin

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D34894

llvm-svn: 306839