[llvm] [RISCV] Support multiple levels of truncates in combineTruncToVnclip. (PR #93752)

[Pengcheng Wang via llvm-commits]

================
@@ -16263,22 +16263,39 @@ static SDValue combineTruncToVnclip(SDNode *N, SelectionDAG &DAG,
     return SDValue();
   };
 
+  SDValue Src = N->getOperand(0);
+
+  // Look through multiple layers of truncates.
+  while (Src.getOpcode() == RISCVISD::TRUNCATE_VECTOR_VL &&
+         Src.getOperand(1) == Mask && Src.getOperand(2) == VL &&
+         Src.hasOneUse())
+    Src = Src.getOperand(0);
+
   SDValue Val;
   unsigned ClipOpc;
-  if ((Val = DetectUSatPattern(N->getOperand(0))))
+  if ((Val = DetectUSatPattern(Src)))
     ClipOpc = RISCVISD::VNCLIPU_VL;
-  else if ((Val = DetectSSatPattern(N->getOperand(0))))
+  else if ((Val = DetectSSatPattern(Src)))
     ClipOpc = RISCVISD::VNCLIP_VL;
   else
     return SDValue();
 
   SDLoc DL(N);
-  // Rounding mode here is arbitrary since we aren't shifting out any bits.
-  return DAG.getNode(
-      ClipOpc, DL, VT,
-      {Val, DAG.getConstant(0, DL, VT), DAG.getUNDEF(VT), Mask,
-       DAG.getTargetConstant(RISCVVXRndMode::RNU, DL, Subtarget.getXLenVT()),
-       VL});
+
+  MVT ValVT = Val.getSimpleValueType();
+
+  do {
+    MVT ValEltVT = MVT::getIntegerVT(ValVT.getScalarSizeInBits() / 2);
+    ValVT = MVT::getVectorVT(ValEltVT, ValVT.getVectorElementCount());
----------------
wangpc-pp wrote:

changeVectorElementType? 
Or we can add a helper `getHalfVectorElementSizeVT` like `getHalfNumVectorElementsVT`?

https://github.com/llvm/llvm-project/pull/93752