PBQP: Minor cleanups to r204857

* Use assignment instead of swap (since the original value is being
  destroyed anyway)

* Rename "updateAdjEdgeId" to "setAdjEdgeId"

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@204983 91177308-0d34-0410-b5e6-96231b3b80d8