Simplifier infelicities

In my travels through the optimiser I discovered two small infelicities

1. In OccurAnal the function occAnalApp was failing to reset occ_encl to OccVanilla. This omission sometimes resulted in over-pessimistic occurrence information.

2. In Simplify.mkDupableAlts we were building a new alternative like

    case e of 
        K a b c -> $j a c

   but with no occ-info on a, c. As a result, if e turned out to be K e1 e2 e3, we'd end up with

     let a = e1; c = e3 in $j a c

   which took another Simplifier iteration to turn into $j e1 e3. I found cases (in compiling GHC itself) where this added entirely-unnecessary Simplifier iterations, which has a bad effect on compiler perf.