[DAGCombine] Add basic optimizations for FREEZE in SelDag

Summary: This patch is the first effort to adding basic optimizations for FREEZE in SelDag.

Reviewers: spatel, lebedev.ri

Reviewed By: spatel

Subscribers: xbolva00, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D76707