nir: Use an integer index for specifying structure fields

Previously, we used a string name.  It was nice for translating out of GLSL
IR (which also does that) but cumbersome the rest of the time.

Reviewed-by: Connor Abbott <cwabbott0@gmail.com>

src/glsl/nir/nir_validate.c

@@ -275,7 +275,7 @@ validate_deref_chain(nir_deref *deref, validate_state *state)
       case nir_deref_type_struct:
          assert(deref->type ==
                 glsl_get_struct_field(parent->type,
-                                      nir_deref_as_struct(deref)->elem));
+                                      nir_deref_as_struct(deref)->index));
          break;
 
       case nir_deref_type_var: