Sink InlineCost.cpp into IPA -- it is now officially an interprocedural
analysis. How cute that it wasn't previously. ;]

Part of this confusion stems from the flattened header file tree. Thanks
to Benjamin for pointing out the goof on IRC, and we're considering
un-flattening the headers, so speak now if that would bug you.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@173033 91177308-0d34-0410-b5e6-96231b3b80d8