vkd3d-shader/hlsl: Add support for sign() intrinsic.

This replicates what seems to be happening when using sign() with shader model 3, which is roughly...

int pos = (int) (x > 0);
int neg = -((int) (x < 0));
int result = pos + neg;

Fixes https://bugs.winehq.org/show_bug.cgi?id=54826