[nvvm] Mark ldu and ldg intrinsics as IntrArgMemOnly.

Summary:
Previously these intrinsics were marked as can-read any memory address.
Now they're marked as reading only the pointer they're passed.

Reviewers: rnk

Subscribers: jholewinski, llvm-commits, tra

Differential Revision: http://reviews.llvm.org/D20080

llvm-svn: 268996