prog_to_nir: fix write from vps to FOG

for fragment programs we already treat fog as a single component value,
but for vp we didn't.

Fixes fog related piglit tests with my out of tree Nouveau nir patches.

Signed-off-by: Karol Herbst <kherbst@redhat.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Eric Anholt <eric@anholt.net>

src/mesa/program/prog_to_nir.c

           */
          src = nir_channel(b, src, 2);
       }
+      if (c->prog->Target == GL_VERTEX_PROGRAM_ARB &&
+          var->data.location == VARYING_SLOT_FOGC) {
+         /* result.fogcoord is a single component value */
+         src = nir_channel(b, src, 0);
+      }
       unsigned num_components = glsl_get_vector_elements(var->type);
       nir_store_var(b, var, src, (1 << num_components) - 1);
    }
       reg->num_components = 4;
 
       nir_variable *var = rzalloc(shader, nir_variable);
-      if (c->prog->Target == GL_FRAGMENT_PROGRAM_ARB && i == FRAG_RESULT_DEPTH)
+      if ((c->prog->Target == GL_FRAGMENT_PROGRAM_ARB && i == FRAG_RESULT_DEPTH) ||
+          (c->prog->Target == GL_VERTEX_PROGRAM_ARB && i == VARYING_SLOT_FOGC))
          var->type = glsl_float_type();
       else
          var->type = glsl_vec4_type();