Remove Nop in simplify_locals.

It's cheap and does not change anything.

compiler/rustc_mir_transform/src/simplify.rs

@@ -569,6 +569,7 @@ fn remove_unused_definitions(used_locals: &mut UsedLocals, body: &mut Body<'_>)
 
                     StatementKind::SetDiscriminant { ref place, .. }
                     | StatementKind::Deinit(ref place) => used_locals.is_used(place.local),
+                    StatementKind::Nop => false,
                     _ => true,
                 };