[NVPTX] noop when kernel pointers are already global

Summary:
Some front ends make kernel pointers global already. In that case,
handlePointerParams does nothing.

Test Plan: more tests in lower-kernel-ptr-arg.ll

Reviewers: grosser

Subscribers: jholewinski, llvm-commits

Differential Revision: http://reviews.llvm.org/D10779

llvm-svn: 240849