[RISCV] Don't run combineBinOp_VLToVWBinOp_VL until after legalize types. NFCI

I noticed this from a discrepancy in fillUpExtensionSupport between how we apparently need to check for legal types for ISD::{ZERO,SIGN}_EXTEND, but we don't need to for RISCVISD::V{Z,S}EXT_VL.

Prior to llvm#72340, combineBinOp_VLToVWBinOp_VL only ran after type legalization because it only operated on _VL nodes.  _VL nodes are only emitted during op legalization, which takes place **after** type legalization, which is presumably why the existing code didn't need to check for legal types.

After llvm#72340 we now handle generic ops like ISD::ADD that exist before op legalization and thus **before** type legalization. This meant that we needed to add extra checks that the narrow type was legal in llvm#76785.

I think the easiest thing to do here is to just maintain the invariant that the types are legal and only run the combine after type legalization.

Author: lukel97

llvm/lib/Target/RISCV/RISCVISelLowering.cpp

@@ -13652,10 +13652,6 @@ struct NodeExtensionHelper {
       unsigned ScalarBits = VT.getScalarSizeInBits();
       unsigned NarrowScalarBits = NarrowVT.getScalarSizeInBits();
 
-      // Ensure the narrowing element type is legal
-      if (!Subtarget.getTargetLowering()->isTypeLegal(NarrowElt.getValueType()))
-        break;
-
       // Ensure the extension's semantic is equivalent to rvv vzext or vsext.
       if (ScalarBits != NarrowScalarBits * 2)
         break;
@@ -13727,14 +13723,11 @@ struct NodeExtensionHelper {
   }
 
   /// Check if \p Root supports any extension folding combines.
-  static bool isSupportedRoot(const SDNode *Root, const SelectionDAG &DAG) {
-    const TargetLowering &TLI = DAG.getTargetLoweringInfo();
+  static bool isSupportedRoot(const SDNode *Root) {
     switch (Root->getOpcode()) {
     case ISD::ADD:
     case ISD::SUB:
     case ISD::MUL: {
-      if (!TLI.isTypeLegal(Root->getValueType(0)))
-        return false;
       return Root->getValueType(0).isScalableVector();
     }
     // Vector Widening Integer Add/Sub/Mul Instructions
@@ -13751,7 +13744,7 @@ struct NodeExtensionHelper {
     case RISCVISD::FMUL_VL:
     case RISCVISD::VFWADD_W_VL:
     case RISCVISD::VFWSUB_W_VL:
-      return TLI.isTypeLegal(Root->getValueType(0));
+      return true;
     default:
       return false;
     }
@@ -13760,9 +13753,10 @@ struct NodeExtensionHelper {
   /// Build a NodeExtensionHelper for \p Root.getOperand(\p OperandIdx).
   NodeExtensionHelper(SDNode *Root, unsigned OperandIdx, SelectionDAG &DAG,
                       const RISCVSubtarget &Subtarget) {
-    assert(isSupportedRoot(Root, DAG) && "Trying to build an helper with an "
-                                         "unsupported root");
+    assert(isSupportedRoot(Root) && "Trying to build an helper with an "
+                                    "unsupported root");
     assert(OperandIdx < 2 && "Requesting something else than LHS or RHS");
+    assert(DAG.getTargetLoweringInfo().isTypeLegal(Root->getValueType(0)));
     OrigOperand = Root->getOperand(OperandIdx);
 
     unsigned Opc = Root->getOpcode();
@@ -13812,7 +13806,7 @@ struct NodeExtensionHelper {
   static std::pair<SDValue, SDValue>
   getMaskAndVL(const SDNode *Root, SelectionDAG &DAG,
                const RISCVSubtarget &Subtarget) {
-    assert(isSupportedRoot(Root, DAG) && "Unexpected root");
+    assert(isSupportedRoot(Root) && "Unexpected root");
     switch (Root->getOpcode()) {
     case ISD::ADD:
     case ISD::SUB:
@@ -14112,8 +14106,10 @@ static SDValue combineBinOp_VLToVWBinOp_VL(SDNode *N,
                                            TargetLowering::DAGCombinerInfo &DCI,
                                            const RISCVSubtarget &Subtarget) {
   SelectionDAG &DAG = DCI.DAG;
+  if (DCI.isBeforeLegalize())
+    return SDValue();
 
-  if (!NodeExtensionHelper::isSupportedRoot(N, DAG))
+  if (!NodeExtensionHelper::isSupportedRoot(N))
     return SDValue();
 
   SmallVector<SDNode *> Worklist;
@@ -14124,7 +14120,7 @@ static SDValue combineBinOp_VLToVWBinOp_VL(SDNode *N,
 
   while (!Worklist.empty()) {
     SDNode *Root = Worklist.pop_back_val();
-    if (!NodeExtensionHelper::isSupportedRoot(Root, DAG))
+    if (!NodeExtensionHelper::isSupportedRoot(Root))
       return SDValue();
 
     NodeExtensionHelper LHS(N, 0, DAG, Subtarget);