Factor out SelectionDAG's switch analysis and lowering into a separate component.

In order for GlobalISel to re-use the significant amount of analysis and
optimization code in SDAG's switch lowering, we first have to extract it and
create an interface to be used by both frameworks.

No test changes as it's NFC.

Differential Revision: https://reviews.llvm.org/D62745

llvm-svn: 362857