Fix Trac #3126: matching overloaded literals

Fix Trac #3126: matching overloaded literals

Claus Reinke uncovered a long-standing bug in GHC, whereby we were
combining the pattern-match on overloaded literals, missing the fact
that an intervening pattern (for a different literal) might also
match.  (If someone had a very odd implementation of fromInteger!)

See Note [Grouping overloaded literal patterns] in Match.lhs

If this merges smoothly to 6.10, go for it, but it's very much
a corner case.

Thank you Claus!