Fix SelectionDAG compile time issue with alias analysis.

Fix SelectionDAG compile time issue with alias analysis.

Add new token factor node and its users to worklist if alias analysis is
turned on, in DAGCombiner::visitTokenFactor(). Alias analysis may cause
a lot of new token factors to be inserted into the DAG, and they need to
be optimized to avoid significant slow-downs.

Reviewed by Hal Finkel.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@228841 91177308-0d34-0410-b5e6-96231b3b80d8