ptx fix for llvm < 3.5
ptx fix for llvm < 3.5


Former-commit-id: 43c7dc558e89dd191b827a9941faf85d53961de7
Trass3r/Halide,Trass3r/Halide,darkbuck/Halide,Trass3r/Halide,Trass3r/Halide,darkbuck/Halide,darkbuck/Halide,Trass3r/Halide,darkbuck/Halide,darkbuck/Halide,Trass3r/Halide,Trass3r/Halide,darkbuck/Halide,darkbuck/Halide