[NFC] Avoid deprecated Type::getPointerTo.
Type::getPointerTo has been deprecated in favour of PointerType::get.
This commit updates the calls accordingly.
Exceptions:
- Some uses were to perform bitcasts between pointer types. These
bitcasts did nothing ever since LLVM moved to opaque pointers, and are
removed instead.
- Some uses were in a context where an IRBuilder was available, in which
case IRBuilder::getPtrTy provides a simpler alternative.