Improve union field access in IR.

[otrho]

Prior to #4336 the IR would index different union fields or variants via the extract_value instruction. This was a bit of a convenient hack as it worked fairly well and didn't require a special extra instruction to do it instead.

This behaviour has translated over to now by done by get_elem_ptr. It's still a hack but no longer necessary. A more proper approach would be to use a cast_ptr instruction, which is what LLVM does.

Care would have to be taken though in ASMgen since we lay out memory in a big-endian format, simply changing the type but not the actual address may not point to the proper field value bytes. (This is an advantage of using little-endian memory!)