MIR-dump: print return type from local_decls for _0

We've kind of got the same information twice in the MIR, between the
return-type field and the local-decls. Seems un-great.

Author: nikomatsakis

src/librustc_mir/util/pretty.rs

@@ -346,7 +346,7 @@ pub fn write_mir_intro<'a, 'gcx, 'tcx>(tcx: TyCtxt<'a, 'gcx, 'tcx>,
     let indented_retptr = format!("{}let mut {:?}: {};",
                                   INDENT,
                                   RETURN_POINTER,
-                                  mir.return_ty);
+                                  mir.local_decls[RETURN_POINTER].ty);
     writeln!(w, "{0:1$} // return pointer",
              indented_retptr,
              ALIGN)?;