ir3: nir_op_f2f16 should round to even

ir3: nir_op_f2f16 should round to even

cat1 instructions round to zero by default.

When fp16 is enabled this will fix:
 dEQP-VK.spirv_assembly.instruction.graphics.float_controls.fp16.input_args.rounding_rte_conv_from_fp32_nostorage_frag
 dEQP-VK.spirv_assembly.instruction.graphics.float_controls.fp16.input_args.rounding_rte_conv_from_fp32_nostorage_vert
 dEQP-VK.spirv_assembly.instruction.compute.float_controls.fp16.input_args.rounding_rte_conv_from_fp32_nostorage

Signed-off-by: Danylo Piliaiev <dpiliaiev@igalia.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/9840>