htools: rewrite Cluster.filterMGResults

filterMGResults was built using a sequence of map and filter calls;
while this was logically correct, it used some incomplete pattern
matching which with the new GHC 7 triggers a warning.

The patch rewrites it using a single foldl that does both the filtering
and the mapping, in a more type-safe way.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>