rather than a bitfield, a great suggestion by Chris during code review.
There is still quite a bit of cruft in the interface, but that requires
sorting out some awkward uses of the cost inside the actual inliner.
No functionality changed intended here.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@153853 91177308-0d34-0410-b5e6-96231b3b80d8