Given that we have two unique keys (guaranteed by Map) checking that
`|range| == 1` is faster.

The fact that `x1 == lo` and `x2 == hi` is guaranteed by mkSwitchTargets
which removes values outside of the range.

Test Plan: ci

Reviewers: bgamari, simonmar

Reviewed By: simonmar

Subscribers: rwbarton, thomie, carter

Differential Revision: https://phabricator.haskell.org/D4295