[AArch64] Fix -Wunused-but-set-variable in GCC non-debug build

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

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D98431

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

@@ -884,8 +884,8 @@ 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
+    (void)KnownValid;
     return ValidCopy;
   };