Fix #10859 by using foldr1 while deriving Eq instances

Summary:
Previously, we were using foldl1 instead, which led to the derived
code to be wrongly associated.

Test Plan: ./validate

Reviewers: RyanGlScott, nomeata, simonpj, bgamari

Reviewed By: RyanGlScott, nomeata

Subscribers: rwbarton, carter

GHC Trac Issues: #10859

Differential Revision: https://phabricator.haskell.org/D5104