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

[Benjamin Maxwell via llvm-commits]

================
@@ -9147,7 +9197,40 @@ 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 (!HasSVECC)
----------------
MacDue wrote:

nit: Can we change `HasSVECC` to something descriptive like `InsertVectorLengthCheck`?

```suggestion
  if (!InsertVectorLengthCheck)
```

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