nir/from_ssa: Don't set reg->parent_instr for ssa_undef instructions
nir/from_ssa: Don't set reg->parent_instr for ssa_undef instructions

Reviewed-by: Jordan Justen <a79006884ba9cf5f4f7fcbb92a15b208780d8c97@intel.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler