Fix empty return with named results

[pelletier]

When a function has named results and uses empty return, the named result would correctly be renamed and hoisted into the frame struct. In that scenario, using an explicit return (such as "return myvalue") works, because the renaming pass goes over the expression in the return statement and replaces the named result with a selector into the frame struct.

However, when using an empty return (just "return"), the named result is not in the AST, and the renaming pass would not see it.

This patch add and final pass to the renaming procedure to change empty returns in functions with named results into explicit returns using selectors into the frame struct. For example:

func example() (out int) {
    out = 42
    return
}

gets rewritten into:

func example() (_fn0 int) {
    var _f0 *struct {
        IP int
        X0 int
    }
    _f0.X0 = 42
    return _f0.X0
}

(other transformations omitted for this example)