Teach steens-aa two things about mod/ref information:

Teach steens-aa two things about mod/ref information:
  1. If memory never escapes the program, it cannot be mod/ref'd by external
     functions.
  2. If memory is global never mod/ref'd in the program, it cannot be mod/ref'd
     by any call.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@20867 91177308-0d34-0410-b5e6-96231b3b80d8