Fix regression in InstCombine introduced by r278944

The intended transform is:
  // Simplify icmp eq (or (ptrtoint P), (ptrtoint Q)), 0
  // -> and (icmp eq P, null), (icmp eq Q, null).

P and Q are both pointer types, but may have different types. We need
two calls to getNullValue() to make the icmps.

llvm-svn: 279271