IR: Change PointerType to derive from Type rather than SequentialType.

As proposed on llvm-dev:
http://lists.llvm.org/pipermail/llvm-dev/2016-October/106640.html

This is for a couple of reasons:

- Values of type PointerType are unlike the other SequentialTypes (arrays
  and vectors) in that they do not hold values of the element type. By moving
  PointerType we can unify certain aspects of how the other SequentialTypes
  are handled.
- PointerType will have no place in the SequentialType hierarchy once
  pointee types are removed, so this is a necessary step towards removing
  pointee types.

Differential Revision: https://reviews.llvm.org/D26595

llvm-svn: 288462