This is just to improve clarity, as it was rather non-obvious
what of the code also applied to classes before.
IrTypeAggr::createInitializerConstant would currently belong in
IrTypeStruct, but this will be refactored anyway.
This also allows us to enable the assert in IrType::IrType.
Unfortunately, this is mostly a "peace of mind" commit, there
doesn't seem to have been a bug actually caused by the transitory
duplicate IrTypePointer/IrTypeStruct instances.
The remaining xyz2llvm static methods are not exactly pretty,
they should probably just be folded into get.
