[DAG] Aggressively cleanup dangling node in CombineZExtLogicopShiftLoad.

While dangling nodes will eventually be pruned when they are
considered, leaving them disables combines requiring single-use.

Reviewers: Carrot, spatel, craig.topper, RKSimon, efriedma

Subscribers: hiraditya, llvm-commits

Differential Revision: https://reviews.llvm.org/D57520

llvm-svn: 352784