CanCover is not transitive. The counter example are Nodes A,B,C such
that CanCover(A, B) and CanCover(B,C) and B is pure. In this case the
effect level of A and B might differ.

This CL adds a missing CanCover check to a case of shift reduction where
we assumed transitivity.

Change-Id: I9f368ffa6907d2af21bbc87b3e6570d0d422e125
Bug: v8:8384
Reviewed-on: https://chromium-review.googlesource.com/c/1307419
Commit-Queue: Sigurd Schneider <sigurds@chromium.org>
Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57157}