[llvm] [SelectionDAG] Use unaligned store to move AVX registers onto stack for `extractelement` (PR #78422)

[Manish Kausik H via llvm-commits]

================
@@ -1425,9 +1426,18 @@ SDValue SelectionDAGLegalize::ExpandExtractFromVectorThroughStack(SDValue Op) {
 
   if (!Ch.getNode()) {
     // Store the value to a temporary stack slot, then LOAD the returned part.
+    auto &MF = DAG.getMachineFunction();
+    auto &MFI = MF.getFrameInfo();
     StackPtr = DAG.CreateStackTemporary(VecVT);
-    Ch = DAG.getStore(DAG.getEntryNode(), dl, Vec, StackPtr,
-                      MachinePointerInfo());
+    int FI = cast<FrameIndexSDNode>(StackPtr.getNode())->getIndex();
+    MachinePointerInfo PtrInfo =
+        MachinePointerInfo::getFixedStack(DAG.getMachineFunction(), FI);
+
+    MachineMemOperand *StoreMMO =
+        MF.getMachineMemOperand(PtrInfo, MachineMemOperand::MOStore,
----------------
Nirhar wrote:

@RKSimon We dont use the `MF.getMachineMemOperand` function call in `ExpandInsertToVectorThroughStack`. If you want I can create a helper function containing:
```
  int FI = cast<FrameIndexSDNode>(StackPtr.getNode())->getIndex();
  MachinePointerInfo PtrInfo =
        MachinePointerInfo::getFixedStack(DAG.getMachineFunction(), FI);
```
Is that okay?

https://github.com/llvm/llvm-project/pull/78422