[LoopPeel] Add EXPENSIVE_CHECKS ifdef guard around domtree verify call

[steplong]

Summary

The verify call was taking 50% of the compile time in our internal LLVM fork when trying to unroll many loops.

[efriedma]

LGTM

You could maybe add a flag to control this, but probably not important.

llvm/lib/Transforms/Utils/LoopPeel.cpp

#endif
   addStringMetadataToLoop(L, PeeledCountMetaData, AlreadyPeeled + PeelCount);
 
   if (Loop *ParentLoop = L->getParentLoop())
     L = ParentLoop;
 
   // We modified the loop, update SE.
   SE->forgetTopmostLoop(L);
 
+#ifdef EXPENSIVE_CHECKS
   // Finally DomtTree must be correct.
   assert(DT->verify(DominatorTree::VerificationLevel::Fast));
+#endif
 
   // FIXME: Incrementally update loop-simplify
   simplifyLoop(L, DT, LI, SE, AC, nullptr, PreserveLCSSA);
 
   NumPeeled++;
 
   return true;
 }