[LICM] Don't verify domtree/loopinfo unless EXPENSIVE_CHECKS is enabled.

[efriedma]

Summary

For large functions, verifying the whole function after each loop takes non-linear time.

[fhahn]

Thanks Eli, LGTM. This is in line with how we handle verification in many other places.

For EXPENSIVE_CHECKS, we also enable verify-loop-info & verify-dom-info, but I think it is worth keeping the extra verification here, as plenty of things could go wrong in LICM after this point, before we are done.

lib/Transforms/Scalar/LICM.cpp

for (Instruction *I : reverse(HoistedInstructions)) {
       }
     }
   }
   if (MSSAU && VerifyMemorySSA)
     MSSAU->getMemorySSA()->verifyMemorySSA();
 
     // Now that we've finished hoisting make sure that LI and DT are still
     // valid.
-#ifndef NDEBUG
+#ifdef EXPENSIVE_CHECKS
   if (Changed) {
     assert(DT->verify(DominatorTree::VerificationLevel::Fast) &&
            "Dominator tree verification failed");
     LI->verify(*DT);
   }
 #endif
 
   return Changed;