[FastISel] Add basic infrastructure to support a target-independent call lowering hook in FastISel. WIP

The infrastructure mimics the call lowering we have already in place for
SelectionDAG, but with limitations. For example structure return demotion and
non-simple types are not supported (yet).

Currently every backend has its own implementation and duplicated code for call
lowering. There is also no specified interface that could be called from
target-independent code. The target-hook is opt-in and doesn't affect current
implementations.