GlobalsAA: Functions with the argmemonly attribute won't read arbitrary globals

Summary:
In preparation for changing GlobalsAA to stop assuming that intrinsics
can't read arbitrary globals, we need to make sure GlobalsAA is querying
function attributes rather than relying on this assumption.

This patch was inspired by: http://reviews.llvm.org/D20206

Reviewers: jmolloy, hfinkel

Subscribers: eli.friedman, llvm-commits

Differential Revision: https://reviews.llvm.org/D21318

llvm-svn: 275433