Use pointer tagging for *T, **T, and ***T. #3684
Description
- Remove the ptrTo field, it won't be necessary anymore.
- Make sure all type structs are 32-bit aligned. This means the lower two bits of the pointer are zero.
- A type *T can then be represented by adding one to the pointer (this is equivalent to setting the lower bit to one).
- A type **T can be represented by adding two to the pointer.
- Same for ***T. A type ****T cannot be represented this way, we may still need a pointer type.
- Kind() returns PointerType when the lower two bits aren't zero, or else it behaves as it already does.
- Elem() subtracts one from the pointer if it is a pointer type, else it works as it already does.