st/mesa: move assert on nr vs insns until after pos_invarient expansion

It is possible to have a 1-instruction vertex shader before expanding
pos_invarient.  Not sure what this assert achieved, but at least move
it where it is correct.

src/mesa/state_tracker/st_atom_shader.c

@@ -287,8 +287,6 @@ find_translated_vp(struct st_context *st,
 #endif
       }
 
-      assert(stvp->Base.Base.NumInstructions > 1);
-
       st_translate_vertex_program(st, stvp, xvp->output_to_slot,
                                   xvp->output_to_semantic_name,
                                   xvp->output_to_semantic_index);