cmm/CBE: Use foldLocalRegsDefd

Simonpj suggested this as a follow-on to #14226 to avoid code
duplication. This also gives us the ability to CBE cases involving
foreign calls for free.

Test Plan: Validate

Reviewers: austin, simonmar, simonpj

Reviewed By: simonpj

Subscribers: michalt, simonpj, rwbarton, thomie

GHC Trac Issues: #14226

Differential Revision: https://phabricator.haskell.org/D3999