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	Fix an interesting case-alternatives filtering bug
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This bug, shown up by Krasimir's ObjectIO suite, caused the
simplifier to encounter a case expression like
	case x of { x:xs -> True; [] -> False }
in a context where x could not possibly be either a (:) or []!
Case expressions in the enclosing scope dealt with it...
So the alternative-filtering removed all the alternatives, leaving
a case expression with no branches, which GHC didn't like one little
bit.

The actual bug was elsewhere; it was because we should sometimes
filter out the DEFAULT alternative, and we weren't doing that.
To fix it, I pulled the alternative-filtering code out of Simplify
and put it in SimplUtils.prepareAlts.  It's nice now.