[NVPTX] Make NVVMReflect a function pass.

Summary:
Currently it's a module pass.  Make it a function pass so that we can
move it to PassManagerBuilder's EP_EarlyAsPossible extension point,
which only accepts function passes.

Reviewers: rnk

Subscribers: tra, llvm-commits, jholewinski

Differential Revision: http://reviews.llvm.org/D18615

llvm-svn: 264919