UnzipArrays doesn't handle arrays of Z

[rrnewton]

I'm in the process of fixing tuple handling (hopefully once and for all), and I tripped across this which seems to be a problem with UnzipArrays.

Corresponds to test p13g.

What happens is that it drops the array entirely, leaving us with an empty list of program results:

unzipETups, output was:
================================================================================

Prog {progBinds = [ProgBind tmp_0
                            (TArray 1 (TTuple []))
                            (SubBinds {subnames = [], arrsize = Just TrivConst 1},
                             (Nothing, KnownSize [1]))
                            (Right Generate (EConst (I 1))
                                            (Lam1 (flatidx0, TInt) (ETuple []))),
                   ProgBind
                            (TTuple [])
                            (SubBinds {subnames = [], arrsize = Nothing},
                             (Nothing, UnknownSize))
                            (Left ETuple [])],
      progResults = WithShapesUnzipped [(tmp_0, [])],
      progType = TArray 1 (TTuple []),
      uniqueCounter = 2,
      typeEnv = [(tmp_0, TArray 1 (TTuple [])),(tmp_0_shape, TTuple [])]}

unzipArrays, output was:
================================================================================

Prog {progBinds = [ProgBind
                            (TArray 1 (TTuple []))
                            (OpInputs [],
                             (SubBinds {subnames = [], arrsize = Just TrivConst 1},
                              (Nothing, KnownSize [1])))
                            (Right Generate (EConst (I 1))
                                            (Lam1 (flatidx0, TInt) (ETuple []))),
                   ProgBind
                            (TTuple [])
                            (OpInputs [],
                             (SubBinds {subnames = [], arrsize = Nothing},
                              (Nothing, UnknownSize)))
                            (Left ETuple [])],
      progResults = WithShapesUnzipped [],
      progType = TArray 1 (TTuple []),
      uniqueCounter = 2,
      typeEnv = [(tmp_0, TArray 1 (TTuple [])),(tmp_0_shape, TTuple [])]}