[NVPTX] Annotate param loads/stores as mayLoad/mayStore.

Summary:
Tablegen was unable to determine that param loads/stores were actually
reading or writing from memory.  I think this isn't a problem in
practice for param stores, because those occur in a block right before
we make our call.  But param loads don't have to at the very beginning
of a function, so should be annotated as mayLoad so we don't incorrectly
optimize them.

Reviewers: jholewinski

Subscribers: jholewinski, llvm-commits

Differential Revision: http://reviews.llvm.org/D17471

llvm-svn: 262381