cmm: Include braces on default branch as required by the parser
cmm: Include braces on default branch as required by the parser

Test Plan: Looking at cmm-dump

Reviewers: bgamari, simonmar

Reviewed By: simonmar

Subscribers: rwbarton, thomie, carter

Differential Revision: https://phabricator.haskell.org/D4293

sdiehl/ghc,sdiehl/ghc,shlevy/ghc,sdiehl/ghc,sdiehl/ghc,shlevy/ghc,shlevy/ghc,shlevy/ghc,sdiehl/ghc,sdiehl/ghc,shlevy/ghc,shlevy/ghc,sdiehl/ghc,shlevy/ghc