nir/algebraic: optimize pack_half_2x16(isnan(a) ? a : a & 0xffffe000, 0)

vkd3d-proton pattern to enforce round-to-zero behaviour with
GLSLstd450PackHalf2x16: https://github.com/HansKristian-Work/dxil-spirv/pull/100

Signed-off-by: Rhys Perry <pendingchaos02@gmail.com>
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