i965/nir: Add utility method for comparisons
i965/nir: Add utility method for comparisons

This method returns the brw_conditional_mod value used when emitting
comparative ALU operations.

It could be moved to brw_nir in the future to reuse it in fs_nir backend.

Reviewed-by: Jason Ekstrand <b186559eaedf698788888c8a2559b4efb4db10d6@intel.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler