Desugaring pattern matches with guards is broken

[benl]

For this program:

main () = f (1, 2)
 
f :: (Int, Int) -> ()
f (a, b) | a == b	= putStr "yep"
f (a, b)		= putStr "no

We get a pattern matching failure instead of "nope".

After the desugarer appends two cases from the two equations for 'f', it still needs to merge the common tuple match.

Test is T61-GuardDesugar