Tidy up IfaceEqualityTyCon

This commit

  commit 85aa1f42
  Date:   Sun Oct 29 20:48:19 2017 -0400
    Fix #14390 by making toIfaceTyCon aware of equality

was a bit over-complicated. This patch simplifies the (horribly
ad-hoc) treatement of IfaceEqualityTyCon, and documents it better.

No visible change in behaviour.