[DAGCombiner] Teach TokenFactor pruning to peek through lifetime nodes

Summary: Lifetime nodes were inhibiting TokenFactor simplification inhibiting chain-based optimizations.

Reviewers: courbet, jyknight

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D59897

llvm-svn: 357121