Don't build unnecessary lets in knownCon

Don't build unnecessary lets in knownCon

Faced with
	case x of y { (a,b) -> rhs }

where x is bound to (c,d), we were generating

	let y = (c,d) in rhs

and thenn hoping to get rid of the y binding by CSE or some such.  It's
better simply not to build it in the first place, by generating

	let y = x in rhs

This patch does the job.