gallivm: passing fp16_split_fp64 to fp16 lowering.
This causes lavapipe to use the split code and fixes accuracy for CTS.
Fixes dEQP-VK.glsl.builtin.precision_fconvert.f64_to_f16*
This causes lavapipe to use the split code and fixes accuracy for CTS.
Fixes dEQP-VK.glsl.builtin.precision_fconvert.f64_to_f16*