patch applied (ghc-6.8/ghc): Derive a valid Ix instance for data Foo = Foo Int Int

[Ian Lynagh]

Sun Mar 30 11:28:13 PDT 2008  Ian Lynagh <igloo at earth.li>
  * Derive a valid Ix instance for data Foo = Foo Int Int
  The old one didn't satisfy the axioms. See trac #2158 for details.

    M ./compiler/typecheck/TcGenDeriv.lhs -1 +5