zink: fix isNan mismatch between NIR and SPIR-V

zink: fix isNan mismatch between NIR and SPIR-V

SPIR-V and Vulkan allow implementations to optimize
float ops assuming inputs are not NaN unless SignedZeroInfNanPreserve
execution mode is enabled.

This means isnan which is turned to nir_op_fneu(a,a) and then emitted
as SpvOpFUnordNotEqual could potentially be optimised to a const 0.

isnan is special in this approach and therefore recognise this pattern,
and emit SpvOpIsNan instead of enabling SignedZeroInfNanPreserve which
will hurt performace of some vulkan drivers.

Fixes dEQP-GL45-ES31.functional.shaders.builtin_functions.common.isnan.*

Cc: mesa-stable
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/18910>