Improve pruning of case alternatives to account for GADTs
Consider
data T a where
T1 :: T Int
T2 :: T Bool
T3 :: T Char
f :: T Bool -> Int
f x = case x of
DEFAULT -> ...
T2 -> 3
Here the DEFAULT case covers multiple constructors (T1,T3), but none
of them can match a scrutinee of type (T Bool). So we can prune away
the default case altogether.
In implementing this, I re-factored this bit of the simplifier, elminiating
prepareAlts from SimplUtils, and putting all the work into simplAlts in
Simplify
The proximate cause was a program written by Manuel using PArrays