Superfluous dyn_cast in DISubprogram::getFunction()

[Duncan P. N. Exon Smith]

> On 2015-Apr-09, at 08:38, Yaron Keren <yaron.keren at gmail.com> wrote:
> 
> This dyn_cast does not seem to be required.
> 
> Yaron
> 
> <cast.diff>

For that matter, the `if (N = get()` above it shouldn't be conditional.

Feel free to commit, but as it happens I'm currently working on deleting
the entire `DIDescriptor` hierarchy so it won't last long anyway ;).