Ben Gamari authored Jan 19, 2016 and Ben Gamari committed Jan 20, 2016

We were previously using `if` in the derivation of `Ix` instances. This
interacts badly with RebindableSyntax as the typechecker doesn't infer
the type of the argument we give to `tagToEnum#`.

Previously we produced, `if (ch >= ah) then (ch <= bh) else False`.

We now produce `(ch >= ah) && (ch <= bh)`

Fixes #11396.

Test Plan: Validate

Reviewers: austin, simonpj

Reviewed By: simonpj

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D1797

GHC Trac Issues: #11396