nir: round f2f16{_rtne/_rtz} correctly for constant expressions

nir: round f2f16{_rtne/_rtz} correctly for constant expressions

As noted in the previous commit, the intermediate cast to float from
double can produce wrong results.

Fixes upcoming Vulkan CTS tests:
dEQP-VK.spirv_assembly.instruction.compute.float_controls.fp16.input_args.rounding_rte_sconst_conv_from_fp64_up
dEQP-VK.spirv_assembly.instruction.compute.float_controls.fp16.input_args.rounding_rte_sconst_conv_from_fp64_up_nostorage
dEQP-VK.spirv_assembly.instruction.graphics.float_controls.fp16.input_args.rounding_rte_sconst_conv_from_fp64_up_vert
dEQP-VK.spirv_assembly.instruction.graphics.float_controls.fp16.input_args.rounding_rte_sconst_conv_from_fp64_up_nostorage_vert
dEQP-VK.spirv_assembly.instruction.graphics.float_controls.fp16.input_args.rounding_rte_sconst_conv_from_fp64_up_frag
dEQP-VK.spirv_assembly.instruction.graphics.float_controls.fp16.input_args.rounding_rte_sconst_conv_from_fp64_up_nostorage_frag

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/25281>