[NVPTX] convert pointers in byval kernel arguments to global

Summary:
For example, in

  struct S {
    int *x;
    int *y;
  };
  __global__ void foo(S s) {
    int *b = s.y;
    // use b
  }

"b" is guaranteed to point to global. NVPTX should emit ld.global/st.global for
accessing "b".

Reviewers: jholewinski

Subscribers: llvm-commits, jholewinski

Differential Revision: http://reviews.llvm.org/D11505

llvm-svn: 243790