Workaround a `-dcore-lint` bug in GHC 7.6/7.7

When compiled with `-dcore-lint`, GHC 7.6/7.7 bails out with a Core lint error;
The workaround/fix used in this commit based on a suggestion given by @simonmar
in GHC #7643.

See also http://ghc.haskell.org/trac/ghc/ticket/8355 for more details.
Merge pull request #1 from hvr/core-lint-fix

Workaround a `-dcore-lint` bug in GHC 7.6/7.7