Be more conservative about duplicating continuations

Be more conservative about duplicating continuations

Roman found that GHC was duplicating continuations that arose (essentially)
from uses of 'seq', or strict constructors.  This fixes the problem;
see the comments mkDupableCont (the Select case with a single alternative).

I'm a little concerned that this may also miss useful case-of-case
tranformations, so I'd like to know if anyone finds that this patch
makes performance worse.

To make it a bit more gung-ho, one could check for all the binders
being dead, before choosing this new, conservative alternative.