SelectionDAG compile time improvement.

One of the phases of SelectionDAG is LegalizeVectors. We don't need to sort the DAG and copy nodes around if there are no vector ops.

Speeds up the compilation time of SelectionDAG on a big scalar workload by ~8%.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@175929 91177308-0d34-0410-b5e6-96231b3b80d8