Ignore arguments when looking for `IndexMut` for subsequent `mut` obligation

Ignore arguments when looking for `IndexMut` for subsequent `mut` obligation

Given code like `v[&field].boo();` where `field: String` and
`.boo(&mut self)`, typeck will have decided that `v` is accessed using
`Index`, but when `boo` adds a new `mut` obligation,
`convert_place_op_to_mutable` is called. When this happens, for *some
reason* the arguments' dereference adjustments are completely ignored
causing an error saying that `IndexMut` is not satisfied:

```
error[E0596]: cannot borrow data in an index of `Indexable` as mutable
  --> src/main.rs:30:5
   |
30 |     v[&field].boo();
   |     ^^^^^^^^^ cannot borrow as mutable
   |
   = help: trait `IndexMut` is required to modify indexed content, but it is not implemented for `Indexable`
```

This is not true, but by changing `try_overloaded_place_op` to retry
when given `Needs::MutPlace` without passing the argument types, the
example successfully compiles.

I believe there might be more appropriate ways to deal with this.