[AArch64] Fix -Wunused-but-set-variable in GCC -DLLVM_ENABLE_ASSERTIONS=off build

Author: MaskRay

llvm/lib/Target/AArch64/GISel/AArch64InstructionSelector.cpp

@@ -865,6 +865,7 @@ static bool selectCopy(MachineInstr &I, const TargetInstrInfo &TII,
 #ifndef NDEBUG
     ValidCopy = KnownValid || isValidCopy(I, DstRegBank, MRI, TRI, RBI);
     assert(ValidCopy && "Invalid copy.");
+    (void)KnownValid;
 #endif
     return ValidCopy;
   };