Optimize redundant vcombine/lo/hi sequences.
Optimize redundant vcombine/lo/hi sequences.


Former-commit-id: e9abb5297a025ff20557291194a216e1cb2ecb55
darkbuck/Halide,darkbuck/Halide,darkbuck/Halide,Trass3r/Halide,darkbuck/Halide,Trass3r/Halide,Trass3r/Halide,Trass3r/Halide,Trass3r/Halide,darkbuck/Halide,Trass3r/Halide,Trass3r/Halide,darkbuck/Halide,darkbuck/Halide