[LegalizeDAG] Use MachinePointerInfo::getUnknownStack in place of MachinePointerInfo() in a couple places. NFC

[craig.topper]

Summary

We know the pointer somewhere on the stack, we just don't know
exactly where since the index may be variable.

[efriedma]

LGTM

llvm/lib/CodeGen/SelectionDAG/LegalizeDAG.cpp

SDValue SelectionDAGLegalize::PerformInsertVectorEltInMemory(SDValue Vec,
   // Store the vector.
   SDValue Ch = DAG.getStore(
       DAG.getEntryNode(), dl, Tmp1, StackPtr,
       MachinePointerInfo::getFixedStack(DAG.getMachineFunction(), SPFI));
 
   SDValue StackPtr2 = TLI.getVectorElementPointer(DAG, StackPtr, VT, Tmp3);
 
   // Store the scalar value.
-  Ch = DAG.getTruncStore(Ch, dl, Tmp2, StackPtr2, MachinePointerInfo(), EltVT);
+  Ch = DAG.getTruncStore(
+      Ch, dl, Tmp2, StackPtr2,
+      MachinePointerInfo::getUnknownStack(DAG.getMachineFunction()), EltVT);
   // Load the updated vector.
   return DAG.getLoad(VT, dl, Ch, StackPtr, MachinePointerInfo::getFixedStack(
                                                DAG.getMachineFunction(), SPFI));
 }
 
 SDValue SelectionDAGLegalize::ExpandINSERT_VECTOR_ELT(SDValue Vec, SDValue Val,
                                                       SDValue Idx,
                                                       const SDLoc &dl) {
SDValue SelectionDAGLegalize::ExpandInsertToVectorThroughStack(SDValue Op) {
 
   // First store the whole vector.
   SDValue Ch = DAG.getStore(DAG.getEntryNode(), dl, Vec, StackPtr, PtrInfo);
 
   // Then store the inserted part.
   SDValue SubStackPtr = TLI.getVectorElementPointer(DAG, StackPtr, VecVT, Idx);
 
   // Store the subvector.
-  Ch = DAG.getStore(Ch, dl, Part, SubStackPtr, MachinePointerInfo());
+  Ch = DAG.getStore(
+      Ch, dl, Part, SubStackPtr,
+      MachinePointerInfo::getUnknownStack(DAG.getMachineFunction()));
 
   // Finally, load the updated vector.
   return DAG.getLoad(Op.getValueType(), dl, Ch, StackPtr, PtrInfo);
 }
 
 SDValue SelectionDAGLegalize::ExpandVectorBuildThroughStack(SDNode* Node) {
   // We can't handle this case efficiently.  Allocate a sufficiently
   // aligned object on the stack, store each element into it, then load