SelectionDAG: Make sure stores are always added to the LegalizedNodes list

When truncated vector stores were being custom lowered in
VectorLegalizer::LegalizeOp(), the old (illegal) and new (legal) node pair
was not being added to LegalizedNodes list.  Instead of the legalized
result being passed to VectorLegalizer::TranslateLegalizeResult(),
the result was being passed back into VectorLegalizer::LegalizeOp(),
which ended up adding a (new, new) pair to the list instead.

This was causing an assertion failure when a custom lowered truncated
vector store was the last instruction a basic block and the VectorLegalizer
was unable to find it in the LegalizedNodes list when updating the
DAG root.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@188953 91177308-0d34-0410-b5e6-96231b3b80d8

lib/CodeGen/SelectionDAG/LegalizeVectorOps.cpp

@@ -171,7 +171,7 @@ SDValue VectorLegalizer::LegalizeOp(SDValue Op) {
         return TranslateLegalizeResults(Op, Result);
       case TargetLowering::Custom:
         Changed = true;
-        return LegalizeOp(TLI.LowerOperation(Result, DAG));
+        return TranslateLegalizeResults(Op, TLI.LowerOperation(Result, DAG));
       case TargetLowering::Expand:
         Changed = true;
         return LegalizeOp(ExpandStore(Op));