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

[simonwallis2]

Summary

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

[MaskRay]

Looks great!

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

auto CheckCopy = [&]() {
     assert((I.isCopy() ||
             (!Register::isPhysicalRegister(I.getOperand(0).getReg()) &&
              !Register::isPhysicalRegister(I.getOperand(1).getReg()))) &&
            "No phys reg on generic operator!");
     bool ValidCopy = true;
 #ifndef NDEBUG
     ValidCopy = KnownValid || isValidCopy(I, DstRegBank, MRI, TRI, RBI);
     assert(ValidCopy && "Invalid copy.");
-    (void)KnownValid;
 #endif
+    (void)KnownValid;
     return ValidCopy;
   };
 
   // Is this a copy? If so, then we may need to insert a subregister copy.
   if (I.isCopy()) {
     // Yes. Check if there's anything to fix up.
     if (!SrcRC) {
       LLVM_DEBUG(dbgs() << "Couldn't determine source register class\n");