Ben Gamari authored May 19, 2018 and Ben Gamari committed May 20, 2018

This was blatantly wrong due to copy-paste blindness:

 * labels were shadowed, which GHC doesn't warn about(!), resulting in
   plainly wrong behavior
 * the sharing check was omitted
 * the wrong closure layout was being used

Moreover, the test wasn't being run due to its primitive dependency, so
I didn't even notice. Sillyness.

Test Plan: install `primitive`, `make test TEST=compact_small_array`

Reviewers: simonmar, erikd

Reviewed By: simonmar

Subscribers: rwbarton, thomie, carter

GHC Trac Issues: #13857.

Differential Revision: https://phabricator.haskell.org/D4702

(cherry picked from commit 12deb9a9)