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

[kuhar]

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
PR33656.

Note that this is only a temporary fix.

lib/IR/Dominators.cpp

bool DominatorTree::isReachableFromEntry(const Use &U) const {
   if (PHINode *PN = dyn_cast<PHINode>(I))
     return isReachableFromEntry(PN->getIncomingBlock(U));
 
   // Everything else uses their operands in their own block.
   return isReachableFromEntry(I->getParent());
 }
 
 void DominatorTree::verifyDomTree() const {
-  if (!verify()) {
+  // Perform the expensive checks only when VerifyDomInfo is set.
+  if (VerifyDomInfo && !verify()) {
     errs() << "\n~~~~~~~~~~~\n\t\tDomTree verification failed!\n~~~~~~~~~~~\n";
     print(errs());
     abort();
   }
 
   Function &F = *getRoot()->getParent();
 
   DominatorTree OtherDT;