Fix bad codegen when prefetching non-uint8 buffers.
Fix bad codegen when prefetching non-uint8 buffers.


Former-commit-id: 02463a3190a1fff3433c25a1140ba3f376bc82e3
darkbuck/Halide,darkbuck/Halide,darkbuck/Halide,darkbuck/Halide,Trass3r/Halide,Trass3r/Halide,darkbuck/Halide,Trass3r/Halide,Trass3r/Halide,darkbuck/Halide,Trass3r/Halide,Trass3r/Halide,Trass3r/Halide,darkbuck/Halide