WIP: nir: split ffma
This will allow us to track better the intent of the original source and enable more optimizations depending on hardware support.
Also this will be useful for OpenCL where the spec requires ffma to be fused and
a * b + c to be unfused except
-cl-mad-enable was added to the compiler options.
in glsl_to_nir we should emit
nir_op_fmadfor hardware without
nir_op_fmadin more drivers supporting it.
Find solution for drivers wanting to support neither, but keep
Find solution for drivers having both, but has a strong preference on one. E.g.
nir_op_fmadbeing faster on some AMD hardware.
nir_op_ffmafirst doing optimizations and merging it back can result in better codegen.