nir/lower_double_ops: relax lower mod()

nir/lower_double_ops: relax lower mod()

Currently when lowering mod() we add an extra instruction so if
mod(a,b) == b then 0 is returned instead of b, as mathematically
mod(a,b) is in the interval [0, b).

But Vulkan spec has relaxed this restriction, and allows the result to
be in the interval [0, b].

This commit takes this in account to remove the extra instruction
required to return 0 instead.

Reviewed-by: Samuel Iglesias Gonsálvez <siglesias@igalia.com>
Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/2922>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/2922>