prog_to_nir: Use nir_op_dph
prog_to_nir: Use nir_op_dph

Shader-db results on HSW:

   instructions in affected programs:     72 -> 56 (-22.22%)

Reviewed-by: Matt Turner <789b315743a28dd066a4ba1459c35951c291d8a6@gmail.com>
Reviewed-by: Kenneth Graunke <bd2562f754ec92342f93f61c25d731e290a2ffa8@whitecape.org>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler