[CFLAA] Don't link GEP pointers to GEP indices.

Code like the following is considered broken, and doesn't need to be
supported by our AA magicks:

void getFoo(int *P) {
  int *PAlias = (int *)((char *)NULL + (uintptr_t)P);
}

This patch makes CFLAA drop support for code like this.

Patch by Jia Chen.

Differential Revision: http://reviews.llvm.org/D20775

llvm-svn: 271322