Note [Combine identical alternatives]

If several alternatives are identical, merge them into a single
DEFAULT alternative.  I've occasionally seen this making a big
difference:

     case e of               =====>     case e of
       C _ -> f x                         D v -> ....v....
       D v -> ....v....                   DEFAULT -> f x
       DEFAULT -> f x

The point is that we merge common RHSs, at least for the DEFAULT case.
[One could do something more elaborate but I've never seen it needed.]
To avoid an expensive test, we just merge branches equal to the *first*
alternative; this picks up the common cases
     a) all branches equal
     b) some branches equal to the DEFAULT (which occurs first)

The case where Combine Identical Alternatives transformation showed up
was like this (base/Foreign/C/Err/Error.hs):

        x | p `is` 1 -> e1
          | p `is` 2 -> e2
        ...etc...

where @is@ was something like

        p `is` n = p /= (-1) && p == n

This gave rise to a horrible sequence of cases

        case p of
          (-1) -> $j p
          1    -> e1
          DEFAULT -> $j p

and similarly in cascade for all the join points!