[NVPTX] Use ldg for explicitly invariant loads.

Summary:
With this change (plus some changes to prevent !invariant from being
clobbered within llvm), clang will be able to model the __ldg CUDA
builtin as an invariant load, rather than as a target-specific llvm
intrinsic.  This will let the optimizer play with these loads --
specifically, we should be able to vectorize them in the load-store
vectorizer.

Reviewers: tra

Subscribers: jholewinski, hfinkel, llvm-commits, chandlerc

Differential Revision: https://reviews.llvm.org/D23477

llvm-svn: 281152