[SelDag] Add FREEZE

Summary:
- Add FREEZE node to SelDag
- Lower FreezeInst (in IR) to FREEZE node
- Add Legalization for FREEZE node

Reviewers: qcolombet, bogner, efriedma, lebedev.ri, nlopes, craig.topper, arsenm

Reviewed By: lebedev.ri

Subscribers: wdng, xbolva00, Petar.Avramovic, liuz, lkail, dylanmckay, hiraditya, Jim, arsenm, craig.topper, RKSimon, spatel, lebedev.ri, regehr, trentxintong, nlopes, mkuper, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D29014