Hoist the two trivial promotion routines out of the big class that

Hoist the two trivial promotion routines out of the big class that
handles the general cases.

The hope is to refactor this so that we don't end up building the entire
class for the trivial cases. I also want to lift a lot of the early
pre-processing in the initial segment of run() into a separate routine,
and really none of it needs to happen inside the primary promotion
class.

These routines in particular used none of the actual state in the
promotion class, so they don't really make sense as members.

llvm-svn: 186781