Refactor case-merging and identical-alternative optimisations

These two optimisations were originally done by SimplUtils.mkCase
*after* all the pieces have been simplified.  Some while ago I
moved them *before*, so they were done by SimplUtils.prepareAlts.
It think the reason was that I couldn't rely on the dead-binder
information on OutIds, and that info is useful in these optimisations.

However,
 (a) Other changes (notably moving case-binder-swap to OccurAnal)
     have meant that dead-binder information is accurate in
     OutIds

 (b) When there is a cascade of case-merges, they happen in
     one sweep if you do it after, but in many sweeps if you
     do it before.  Reason: doing it after means you are looking
     at nice simplified Core.