propagating unreachable for functions returning never type

Signed-off-by: Jakub Doka <jakub.doka2@gmail.com>

lang/src/son.rs

@@ -2660,6 +2660,10 @@ impl<'a> Codegen<'a> {
 
             self.add_clobber_stores(clobbered_aliases);
 
+            if sig.ret == ty::Id::NEVER {
+                return None;
+            }
+
             alt_value.or(Some(Value::new(self.ci.ctrl.get()).ty(sig.ret)))
         }
     }