nir/cf: add insert_phi_undef() helper
nir/cf: add insert_phi_undef() helper

Signed-off-by: Connor Abbott <99c5849d46b9d5d9781f7baa7735ff66305c63a7@intel.com>
Reviewed-by: Kenneth Graunke <bd2562f754ec92342f93f61c25d731e290a2ffa8@whitecape.org>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler