Improve comments in CmmSwitch
Improve comments in CmmSwitch

addressing some valuable feedback by thomie at
https://phabricator.haskell.org/rGHCde1160be0477

Differential Revision: https://phabricator.haskell.org/D1816

(cherry picked from commit b617e9fe9ce22fb108b7e4a6694167dd893b9dfc)

GaloisInc/halvm-ghc,GaloisInc/halvm-ghc,GaloisInc/halvm-ghc,GaloisInc/halvm-ghc,GaloisInc/halvm-ghc,GaloisInc/halvm-ghc,GaloisInc/halvm-ghc