i8 and i16 integer overflow is well-defined in Halide
i8 and i16 integer overflow is well-defined in Halide

This will hopefully fix some simd op check failures on arm where LLVM is
making optimizations that we would consider invalid because we're told
it i16 and i8 math doesn't overflow.


Former-commit-id: ac4ff474ccb685a0a16fa309d79a35a2dbcb8a2c
darkbuck/Halide,Trass3r/Halide,darkbuck/Halide,Trass3r/Halide,darkbuck/Halide,darkbuck/Halide,Trass3r/Halide,darkbuck/Halide,darkbuck/Halide,Trass3r/Halide,darkbuck/Halide,Trass3r/Halide,Trass3r/Halide,Trass3r/Halide