Fix simplification of pointer-object comparison [blocks: #4628]

[tautschnig]

1. pointer_object((T1 *)NULL) equals pointer_object((T2 *)NULL) for any
   types T1, T2. Previously, this would return false, unless T1==T2.
2. Do not restrict the above to NULL, but instead let the existing logic
   in simplify_inequality fully simplify this.
3. Add a unit test of this code, which highlighted further bugs and
   limitations: the unit test previously did not set the instance of the
   desired dynamic object, and address-of inequalities over dynamic objects
   can also be simplified.

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[smowton]

The unit test doesn't currently cover all the added behaviour (e.g. the line @owen-jones-diffblue suggested you change)

[tautschnig]

Unit test extended (and locally tested that it does fail without @owen-jones-diffblue's fix).

[smowton]

Thought for the future: in this case I think we have a very particular simplification in mind -- presumably that nulls are typed so the cast might be folded, and that (T*)(S*)x can become (T*)x. However bluntly using the simplifier always risks that a much more expensive transformation might be attempted. This suggests factoring out particular simplifications as utility methods, used both during simplification and as the routine way of creating a pointer cast.

[tautschnig]

That certainly is something to review all the simplifier code for. I actually have some patches in a branch already, but it's not quite a systematic review yet. Will work on this in a separate PR.