ir: simplify_standard_types should be recursive.

Stuff like *mut Option<&mut Foo> should really be *mut *mut Foo for
example.

tests/rust/simplify_option_ptr.rs

@@ -24,4 +24,5 @@ pub extern "C" fn root(
     c: Foo,
     d: Bar,
     e: *mut Option<*mut Opaque>,
+    f: extern "C" fn(Option<&Opaque>),
 ) { }