Silence warning in assert introduced in rL349973.

lib/Target/X86/X86DiscriminateMemOps.cpp

for (auto &MI : MBB) {
         }
         // Since we were able to encode, bump the MemOpDiscriminators.
         ++MemOpDiscriminators[L];
         DI = DI->cloneWithDiscriminator(EncodedDiscriminator.getValue());
         updateDebugInfo(&MI, DI);
         Changed = true;
         std::pair<DenseSet<unsigned>::iterator, bool> MustInsert =
             Set.insert(DI->getBaseDiscriminator());
+        (void)MustInsert; // Silence warning in release build.
         assert(MustInsert.second && "New discriminator shouldn't be present in set");
       }
 
       // Bump the reference DI to avoid cramming discriminators on line 0.
       // FIXME(mtrofin): pin ReferenceDI on blocks or first instruction with DI
       // in a block. It's more consistent than just relying on the last memop
       // instruction we happened to see.
       ReferenceDI = DI;
     }
   }
   return Changed;
 }
 
 FunctionPass *llvm::createX86DiscriminateMemOpsPass() {
   return new X86DiscriminateMemOps();
 }