Summary:
`varSetElemsWellScoped` uses `varSetElems` under the hood which
introduces unnecessary nondeterminism.
This does the same thing, possibly cheaper, while preserving
determinism.

Test Plan: ./validate

Reviewers: simonmar, goldfire, austin, bgamari, simonpj

Reviewed By: simonpj

Subscribers: thomie, RyanGlScott

Differential Revision: https://phabricator.haskell.org/D2116

GHC Trac Issues: #4012