ir: Avoid generating bogus pointer arguments.

Now that we track nullability in the pointer type it's easy to do it.

Fixes #223

tests/rust/simplify_option_ptr.rs

@@ -6,6 +6,7 @@ struct Foo {
     x: Option<&Opaque>,
     y: Option<&mut Opaque>,
     z: Option<fn () -> ()>,
+    zz: *mut Option<fn () -> ()>,
 }
 
 #[repr(C)]
@@ -13,6 +14,7 @@ union Bar {
     x: Option<&Opaque>,
     y: Option<&mut Opaque>,
     z: Option<fn () -> ()>,
+    zz: *mut Option<fn () -> ()>,
 }
 
 #[no_mangle]
@@ -20,5 +22,6 @@ pub extern "C" fn root(
 	a: Option<&Opaque>,
     b: Option<&mut Opaque>,
     c: Foo,
-    d: Bar
+    d: Bar,
+    e: *mut Option<*mut Opaque>,
 ) { }