[InstCombine] Fix a bug when combining `icmp` from `ptrtoint`

- First, there's a crash when we try to combine that pointers into `icmp`
  directly by creating a `bitcast`, which is invalid if that two pointers are
  from different address spaces.

- It's not always appropriate to cast one pointer to another if they are from
  different address spaces as that is not no-op cast. Instead, we only combine
  `icmp` from `ptrtoint` if that two pointers are of the same address space.

llvm-svn: 229063