Use -ffast-math... if you dare. Or just use
inline double position_from_steps(int steps) {
return (2.0 * PI / 4096.0) * steps;
};
The reason it doesn't optimise it into one multiplication is because that isn't actually the same calculation. Floating point numbers aren't real numbers, so it isn't true that (a * b) / c == (a / c) * b. Since the "optimisation" would actually change the semantics, compilers don't do it by default.
-ffast-math says "hey I don't care about reliable semantics; just do whatever to make it fast". But it also does a load of stuff that you may find surprising. so you really shouldn't use it. Much better just to reorganise your code.
You can also use -funsafe-math-optimizations which is the more specific subset of -ffast-math that does this particular optimisation. See https://gcc.gnu.org/wiki/FloatingPointMath
Also I would question if it matters. This micro-optimisation will make zero difference on x86. On a microcontroller (I assume what you're actually using if you're encoding motor positions) it might matter a bit more but double check your microcontroller even has an FPU. Lots don't. Even if it does, doing it in integer space will probably be faster (though not necessarily).