[AArch64] Silence a warning (NFC)

/llvm-project/llvm/lib/Target/AArch64/MachineSMEABIPass.cpp:952:12:
 error: unused variable 'SMEFnAttrs' [-Werror,-Wunused-variable]
  SMEAttrs SMEFnAttrs = AFI->getSMEFnAttrs();
           ^
1 error generated.

Author: DamonFool

llvm/lib/Target/AArch64/MachineSMEABIPass.cpp

@@ -949,7 +949,7 @@ void MachineSMEABI::emitStateChange(EmitContext &Context,
   if (From == ZAState::ENTRY && To == ZAState::OFF)
     return;
 
-  SMEAttrs SMEFnAttrs = AFI->getSMEFnAttrs();
+  [[maybe_unused]] SMEAttrs SMEFnAttrs = AFI->getSMEFnAttrs();
 
   // TODO: Avoid setting up the save buffer if there's no transition to
   // LOCAL_SAVED.