`ref mut` check for mutable argument not done on methods

Related to #7623.

E.g., this compiles without errors;

```sway
script;

struct S {}

impl S {
    fn method(self, ref mut _x: u64) { }
} 

fn main() {
    let x = 42u64;       // <<<<<----- `x` is not mutable...
    S {}.method(x);      // <<<<<----- ...but is passed without errors.
}
```

The check is properly done on associated functions and module functions.