[CFLAA] Handle global/arg attrs more sanely.

[CFLAA] Handle global/arg attrs more sanely.

Prior to this patch, we used argument/global stratified attributes in
order to note that a value could have come from either dereferencing a
global/arg, or from the assignment from a global/arg.

Now, AttrUnknown is placed on sets when we see a dereference, instead of
the global/arg attributes. This allows us to be more aggressive in the
future when we see global/arg attributes without AttrUnknown.

Patch by Jia Chen.

Differential Revision: http://reviews.llvm.org/D21110

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@272335 91177308-0d34-0410-b5e6-96231b3b80d8