Simplify trivial casts (and handle pointers specially).

Simplify trivial casts (and handle pointers specially).

This does trivial simplification of casting to the same
typesize. HOWEVER. We split casts up into whether they
cast to a dereferencable pointer type or not, and we don't
simplify pointer casts. This should mean that if we
ever want to do type-based alias analysis, we can still
avoid casted accesses.

(If we do type-based alias analysis, we'll also need to make
a union access do a cast. Which we probably should do anyway).