Fix build warnings introduced in rL347938

[mtrofin]

Summary

Suppressed warnings in release builds due to variable used
only in assert statement.

lib/Target/X86/X86DiscriminateMemOps.cpp

for (auto &MI : MBB) {
       const std::pair<DenseSet<unsigned>::iterator, bool> TryInsert =
           Set.insert(DI->getBaseDiscriminator());
       if (!TryInsert.second) {
         DI = DI->setBaseDiscriminator(++MemOpDiscriminators[diToLocation(DI)]);
         updateDebugInfo(&MI, DI);
         Changed = true;
         const std::pair<DenseSet<unsigned>::iterator, bool> MustInsert =
             Set.insert(DI->getBaseDiscriminator());
-        assert(MustInsert.second);
+        (void)MustInsert; // silence warning.
+        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();
 }