prog_to_nir: Use nir_builder_insert() rather than poking at cf_list.
prog_to_nir: Use nir_builder_insert() rather than poking at cf_list.

I intend to remove nir_builder::cf_node_list, so I can't have this code
poking at it directly.  The proper way is to set the insertion point and
then simply insert things there.

Signed-off-by: Kenneth Graunke <bd2562f754ec92342f93f61c25d731e290a2ffa8@whitecape.org>
Reviewed-by: Eric Anholt <96f164ad4d9b2b0dacf8ebee2bb1eeb3aa69adf1@anholt.net>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler