Fix occAnalApp

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

I tripped over this when analysing eta-expansions.

Compile times in perf/compiler fell slightly (no increases)

  PmSeriesG(normal) ghc/alloc    50738104.0    50580440.0  -0.3%
  PmSeriesS(normal) ghc/alloc    64045284.0    63739384.0  -0.5%
  PmSeriesT(normal) ghc/alloc    94430324.0    93800688.0  -0.7%
  PmSeriesV(normal) ghc/alloc    63051056.0    62758240.0  -0.5%
     T10547(normal) ghc/alloc    29322840.0    29307784.0  -0.1%
     T10858(normal) ghc/alloc   191988716.0   189801744.0  -1.1%
     T11195(normal) ghc/alloc   282654016.0   281839440.0  -0.3%
     T11276(normal) ghc/alloc   142994648.0   142338688.0  -0.5%
    T11303b(normal) ghc/alloc    46435532.0    46343376.0  -0.2%
     T11374(normal) ghc/alloc   256866536.0   255653056.0  -0.5%
     T11822(normal) ghc/alloc   140210356.0   138935296.0  -0.9%
     T12234(optasm) ghc/alloc    60753880.0    60720648.0  -0.1%
       T14052(ghci) ghc/alloc  2235105796.0  2230906584.0  -0.2%
     T17096(normal) ghc/alloc   297725396.0   296237112.0  -0.5%
     T17836(normal) ghc/alloc  1127785292.0  1125316160.0  -0.2%
    T17836b(normal) ghc/alloc    54761928.0    54637592.0  -0.2%
     T17977(normal) ghc/alloc    47529464.0    47397048.0  -0.3%
    T17977b(normal) ghc/alloc    42906972.0    42809824.0  -0.2%
     T18478(normal) ghc/alloc   777385708.0   774219280.0  -0.4%
    T18698a(normal) ghc/alloc   415097664.0   409009120.0  -1.5% GOOD
    T18698b(normal) ghc/alloc   500082104.0   493124016.0  -1.4% GOOD
     T18923(normal) ghc/alloc    72252364.0    72216016.0  -0.1%
      T1969(normal) ghc/alloc   811581860.0   804883136.0  -0.8%
      T5837(normal) ghc/alloc    37688048.0    37666288.0  -0.1%

Nice!

Metric Decrease:
    T18698a
    T18698b