Summary:
This allows the code generator to give hints to later code generation
steps about which branch is most likely to be taken.  Right now it
is only taken into account in one place: a special case in
CmmContFlowOpt that swapped branches over to maximise the chance of
fallthrough, which is now disabled when there is a likelihood setting.

Test Plan: validate

Reviewers: austin, simonpj, bgamari, ezyang, tibbe

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D1273