cmmCreateSwitchPlan: Handle singletons up-front

Authored by nomeata.

Description

cmmCreateSwitchPlan: Handle singletons up-front

and make sure these are implemented with an equality check, which is a
shorter instruction. This was suggested by rwbarton in Trac #10677.

Differential Revision: https://phabricator.haskell.org/D1137

Details