1. The divisor is not actually a compile-time constant, but comes from a small set. You can compute a table of corresponding constants and use the same code for every divisor.
2. You want to do many divisions by many different divisors (whether known at compile-time or drawn from a small set, like above, or pre-computable and applied many times... think of something like a quantization matrix for image coding, for example). Here Robison easily SIMDifies, whereas an algorithm that needs different code paths for every divisor is going to be much more difficult.
Regarding the lack of native signed-unsigned multiplies, as someone who does a lot of multimedia processing, I find the set of available multiplication instructions on x86 and ARM to be rather shamefully limited (especially x86). I recall a colleague who used to optimize for VideoCore (the GPU in a Raspberry Pi) saying that when he first started he was astounded that it had 80 different multiply instructions, but within a year or two he had used basically all of them.