[GlobalsAA] Relax condition in checking globals as args to functions

Summary:
Since globals may escape as function arguments (even when they have been 
found to be non-escaping, because of optimizations such as memcpyoptimizer
that replaces stores with memcpy), all arguments to a function are checked
during query to make sure they are identifiable. At that time, also ensure
we return a conservative result only if the arguments don't alias to our global.

Reviewers: hfinkel, jmolloy

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D16140

llvm-svn: 257750