Split prefetch_box and inject_prefetch_stmts out of For visitor
Split prefetch_box and inject_prefetch_stmts out of For visitor


Former-commit-id: 57b10030f04e80afe84d440aee09a42fddd9731d
darkbuck/Halide,Trass3r/Halide,Trass3r/Halide,darkbuck/Halide,Trass3r/Halide,Trass3r/Halide,darkbuck/Halide,darkbuck/Halide,darkbuck/Halide,Trass3r/Halide,Trass3r/Halide,darkbuck/Halide,Trass3r/Halide,darkbuck/Halide