[llvm] [AArch64][SME] Introduce CHECK_MATCHING_VL pseudo for streaming transitions (PR #157510)

[Sander de Smalen via llvm-commits]

================
@@ -9147,7 +9199,38 @@ SDValue AArch64TargetLowering::changeStreamingMode(SelectionDAG &DAG, SDLoc DL,
   if (InGlue)
     Ops.push_back(InGlue);
 
-  return DAG.getNode(Opcode, DL, DAG.getVTList(MVT::Other, MVT::Glue), Ops);
+  if (!InsertVectorLengthCheck)
+    return DAG.getNode(Opcode, DL, DAG.getVTList(MVT::Other, MVT::Glue), Ops);
+
+  auto GetCheckVL = [&](SDValue Chain, SDValue InGlue = SDValue()) -> SDValue {
+    SmallVector<SDValue, 2> Ops = {Chain};
+    if (InGlue)
+      Ops.push_back(InGlue);
+    return DAG.getNode(AArch64ISD::CHECK_MATCHING_VL, DL,
+                       DAG.getVTList(MVT::Other, MVT::Glue), Ops);
+  };
+
+  // Non-streaming -> Streaming
+  if (Enable) {
+    SDValue CheckVL = GetCheckVL(Chain, InGlue);
+
+    // Replace chain
+    Ops[0] = CheckVL.getValue(0);
+
+    // Replace/append glue
+    if (InGlue)
+      Ops.back() = CheckVL.getValue(1);
+    else
+      Ops.push_back(CheckVL.getValue(1));
+
+    return DAG.getNode(Opcode, DL, DAG.getVTList(MVT::Other, MVT::Glue), Ops);
+  }
+
+  // Streaming -> Non-streaming
----------------
sdesmalen-arm wrote:

nit: could you add a brief comment explaining why this must be done _after_ the `smstop` ?

https://github.com/llvm/llvm-project/pull/157510