Initial commit by Amin Timany.

I have introduced the following definition to avoid many case
analyses where both branches had nearly identical proofs.

Definition uPred_always_if {M} (p : bool) (P : uPred M) : uPred M :=
  (if p then □ P else P)%I.

And make constants P for which we do not want of_val P to reduce Opaque.

through definitions.

This is more consistent with wp_cas_fail.

We may want to do the same for iIntros ? and iDestruct "H" as {?} "H",
but that requires more work. However, I do not think I want to rely on
names chosen this way.