[RISCV] Reason explicitly about mask and rounding mode in performComb…

…ineVMergeAndVOps [nfc]

This is a subset of Luke's D155063.  I'm splitting pieces and landing them in the process of convincing myself all the individual transforms are in fact correct.

The code structure here is overly verbose.  I'm landing this staging change with the code structure exactly matching the non-masked case to make the following cleanup that commons this all obviously correct.

llvm/lib/Target/RISCV/RISCVISelDAGToDAG.cpp

@@ -3318,6 +3318,14 @@ bool RISCVDAGToDAGISel::performCombineVMergeAndVOps(SDNode *N) {
         !True->getFlags().hasNoFPExcept())
       return false;
 
+  // From the preconditions we checked above, we know the mask and thus glue
+  // for the result node will be taken from True.
+  if (IsMasked) {
+    Mask = True->getOperand(Info->MaskOpIdx);
+    Glue = True->getOperand(True->getNumOperands() - 1);
+    assert(Glue.getValueType() == MVT::Glue);
+  }
+
   SDLoc DL(N);
   unsigned MaskedOpc = Info->MaskedPseudo;
 #ifndef NDEBUG
@@ -3340,7 +3348,19 @@ bool RISCVDAGToDAGISel::performCombineVMergeAndVOps(SDNode *N) {
   SmallVector<SDValue, 8> Ops;
   if (IsMasked) {
     Ops.push_back(False);
-    Ops.append(True->op_begin() + 1, True->op_begin() + TrueVLIndex);
+    if (RISCVII::hasRoundModeOp(TrueTSFlags)) {
+      // For masked "VOp" with rounding mode operand, that is interfaces like
+      // (..., vm, rm, vl, policy).
+      // Check the rounding mode pseudo nodes under RISCVInstrInfoVPseudos.td
+      SDValue RoundMode = True->getOperand(TrueVLIndex - 1);
+      Ops.append(True->op_begin() + HasTiedDest,
+                 True->op_begin() + TrueVLIndex - 2);
+      Ops.append({Mask, RoundMode});
+    } else {
+      Ops.append(True->op_begin() + HasTiedDest,
+                 True->op_begin() + TrueVLIndex - 1);
+      Ops.push_back(Mask);
+    }
   } else {
     Ops.push_back(False);
     if (RISCVII::hasRoundModeOp(TrueTSFlags)) {
@@ -3365,11 +3385,6 @@ bool RISCVDAGToDAGISel::performCombineVMergeAndVOps(SDNode *N) {
     Ops.push_back(True.getOperand(TrueChainOpIdx));
 
   // Add the glue for the CopyToReg of mask->v0.
-  if (IsMasked) {
-    // Matches the Merge operand above
-    assert(True->getGluedNode());
-    Glue = True->getOperand(True->getNumOperands() - 1);
-  }
   Ops.push_back(Glue);
 
   SDNode *Result =