Andrew Farmer authored May 21, 2016 and Ben Gamari committed May 21, 2016

Deriving clauses (Ord especially) generated if-expressions with nlHsIf
which were subject to RebindableSyntax. This changes nlHsIf to generate
concrete if-expressions.

There was also an error about calling tagToEnum# at a polymorphic type,
which is not allowed. Fixing nlHsIf didn't fix this for some reason, so
I generated a type ascription around the call to tagToEnum#. Not sure
why the typechecker could not figure this out.

Test Plan: Added a test, ran validate.

Reviewers: simonpj, simonmar, austin, bgamari

Reviewed By: bgamari

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D2247

GHC Trac Issues: #12080