[DAGCombine] Prune unnused nodes.

Summary:
Nodes that have no uses are eventually pruned when they are selected
from the worklist. Record nodes newly added to the worklist or DAG and
perform pruning after every combine attempt.

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

Reviewed By: jyknight

Subscribers: jdoerfert, jyknight, nemanjai, jvesely, nhaehnle, javed.absar, hiraditya, jsji, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D58070

llvm-svn: 357283