[IR] Remove LLVMPointerToElt and LLVMAnyPointerToElt intrinsic types (NFC)

With opaque pointers, LLVMPointerToElt can be replaced by llvm_ptr_ty
and LLVMAnyPointerToElt by llvm_anyptr_ty.

This still leaves LLVMVectorOfAnyPointersToElt, where we can't just
replace with an existing IIT descriptor.

Differential Revision: https://reviews.llvm.org/D155167