rwbarton authored Jan 23, 2017 and Ben Gamari committed Jan 23, 2017

Commit d3b546b1 added a "scrutinee constant folding" pass
that rewrites a case expression whose scrutinee is an expression like
x +# 3#. But case expressions are supposed to have their alternatives in
sorted order, so when the scrutinee is (for example) negateInt# x#, we
need to re-sort the alternatives after mapping their values.

This showed up as a core lint failure when compiling System.Process.Posix:

    isSigIntQuit n = sig == sigINT || sig == sigQUIT
        where sig = fromIntegral (-n)

Data.List.sortBy is supposed to be linear-time on sorted or reverse-sorted
input, so it is probably not worth doing anything more clever than this.

Test Plan: Added a new test T13170 for the above case.

Reviewers: austin, hsyl20, simonpj, bgamari

Reviewed By: hsyl20, simonpj, bgamari

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D3008

GHC Trac Issues: #13170