don't warn when removing all result parameters would add complexity to the code

[mvdan]

For example, say that the result of foo here is never used, as it's always called like foo() or _ = foo():

func foo() bar {
    if cond {
        return foo2()
    }
    return foo3()
}

However, following unparam's advice to remove the result parameter will make the code longer:

func foo() {
    if cond {
        foo2()
        return
    }
    foo3()
}

A couple of examples from go version devel +4a52452a2f Wed Feb 28 22:05:23 2018 +0000 linux/amd64:

regexp/syntax/parse.go:297:27: (*parser).concat - result 0 (*regexp/syntax.Regexp) is never used
regexp/syntax/parse.go:317:30: (*parser).alternate - result 0 (*regexp/syntax.Regexp) is never used

[mvdan]

The rule here should probably be "can all return call() lines be rewritten as call() without changing the behavior?". That will only be true if there is a single return, and it is at the end of the main block.