[SelectionDAG] When splitting scatter nodes during DAGCombine, create a serial chain dependency.

Scatter could have multiple identical indices. We need to maintain sequential order. We get this right in LegalizeVectorTypes, but not in this code.

Differential Revision: https://reviews.llvm.org/D50374

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@339157 91177308-0d34-0410-b5e6-96231b3b80d8