Switch Pipeline to use LLVM global if const size
Switch Pipeline to use LLVM global if const size

This may be more efficient in general, and it allows these buffers
to live in CUDA shared memory.

Former-commit-id: 8c5dc1bf17b31000d23a459d6ff08908fd46264e
Trass3r/Halide,darkbuck/Halide,Trass3r/Halide,Trass3r/Halide,Trass3r/Halide,darkbuck/Halide,darkbuck/Halide,darkbuck/Halide,darkbuck/Halide,Trass3r/Halide,darkbuck/Halide,Trass3r/Halide,Trass3r/Halide,darkbuck/Halide