Add likely annotation to cmm files in a few obvious places.
Provide information about paths more likely to be taken in the cmm files
used by the rts.
This leads to slightly better assembly being generated.
Reviewers: bgamari, erikd, simonmar
Subscribers: alexbiehl, rwbarton, thomie, carter
GHC Trac Issues: Trac #14672
Differential Revision: https://phabricator.haskell.org/D4324