Switched to sm_20 to work with trunk llvm.
Switched to sm_20 to work with trunk llvm.


Former-commit-id: 3540ff0d67040dfbc8874382cead9d6598adf97a
darkbuck/Halide,Trass3r/Halide,Trass3r/Halide,Trass3r/Halide,Trass3r/Halide,darkbuck/Halide,darkbuck/Halide,darkbuck/Halide,Trass3r/Halide,Trass3r/Halide,Trass3r/Halide,darkbuck/Halide,darkbuck/Halide,darkbuck/Halide