- Use the existing utility function for computing the primitive
   restart index.
 - Fix bad merge that removed the unfoldShortCircuits feature.

BUG=1017337
BUG=angleproject:3997

Change-Id: I4ba5ada9816731bdd29a0e518ce225bdd69a762e
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1876453Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Jonah Ryan-Davis <jonahr@google.com>
Commit-Queue: Geoff Lang <geofflang@chromium.org>