Persuade GCC that there is nothing worth warning about here (there isn't).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@149834 91177308-0d34-0410-b5e6-96231b3b80d8

lib/CodeGen/MachineCSE.cpp

@@ -255,6 +255,7 @@ bool MachineCSE::PhysRegDefsReach(MachineInstr *CSMI, MachineInstr *MI,
 
     if (I == EE) {
       assert(CrossMBB && "Reaching end-of-MBB without finding MI?");
+      (void)CrossMBB;
       CrossMBB = false;
       NonLocal = true;
       I = MBB->begin();