Revert "Avoid unnecessary temporary on assignments"

This reverts commit a0ec902e239b2219edf1a18b036dd32c18d3be42.

Leaving out the temporary for the functions return value can lead to a
situation that conflicts with rust's aliasing rules.

Given this:

````rust
fn func(f: &mut Foo) -> Foo { /* ... */ }

fn bar() {
    let mut foo = Foo { /* ... */ };

    f = func(&mut f);
}
````

We effectively get two mutable references to the same variable `foo` at
the same time. One for the parameter `f`, and one for the hidden
out-pointer. So we can't just `trans_into` the destination directly, but
must use `trans` to get a new temporary slot from which the result can
be copied.