[RISCV][DAGCombiner] Fix missed combines in combineOp_VLToVWOp_VL

The previous implementation of combineOp_VLToVWOp_VL manually replaced old
nodes with newly created widened nodes, but only added the new node itself to
the DAGCombiner worklist. Since the users of the new node were not added,
some combine opportunities could be missed when external DAGCombiner passes
expected those users to be reconsidered.

This patch replaces the custom replacement logic with a call to
DCI.CombineTo(), which performs node replacement in a way consistent with
DAGCombiner::Run:
- Replace all uses of the old node.
- Add the new node and its users to the worklist.
- Clean up unused nodes when appropriate.

Using CombineTo ensures that combineOp_VLToVWOp_VL behaves consistently with
the standard DAGCombiner update model, avoiding discrepancies between the
private worklist inside this routine and the global worklist managed by the
combiner.

This resolves missed combine cases involving VL -> VW operator widening.

Author: Kai Lin

llvm/lib/Target/RISCV/RISCVISelLowering.cpp

@@ -18300,8 +18300,7 @@ static SDValue combineOp_VLToVWOp_VL(SDNode *N,
     }
   }
   for (std::pair<SDValue, SDValue> OldNewValues : ValuesToReplace) {
-    DAG.ReplaceAllUsesOfValueWith(OldNewValues.first, OldNewValues.second);
-    DCI.AddToWorklist(OldNewValues.second.getNode());
+    DCI.CombineTo(OldNewValues.first.getNode(), OldNewValues.second);
   }
   return InputRootReplacement;
 }

llvm/test/CodeGen/RISCV/rvv/combine-vl-vw-macc.ll

@@ -7,19 +7,16 @@ define void @matmul_min(ptr %vptr, ptr %scalars, ptr %acc0_ptr, ptr %acc1_ptr) {
 ; CHECK:       # %bb.0: # %entry
 ; CHECK-NEXT:    li a4, 64
 ; CHECK-NEXT:    li a5, 32
-; CHECK-NEXT:    vsetvli zero, a4, e8, m4, ta, ma
-; CHECK-NEXT:    vle8.v v8, (a2)
 ; CHECK-NEXT:    vsetvli zero, a5, e8, m2, ta, ma
-; CHECK-NEXT:    vle8.v v20, (a0)
+; CHECK-NEXT:    vle8.v v16, (a0)
 ; CHECK-NEXT:    lb a0, 0(a1)
 ; CHECK-NEXT:    lb a1, 1(a1)
 ; CHECK-NEXT:    vsetvli zero, a4, e8, m4, ta, ma
+; CHECK-NEXT:    vle8.v v8, (a2)
 ; CHECK-NEXT:    vle8.v v12, (a3)
 ; CHECK-NEXT:    vsetvli zero, a5, e8, m2, ta, ma
-; CHECK-NEXT:    vwmacc.vx v8, a0, v20
-; CHECK-NEXT:    vwmul.vx v16, v20, a1
-; CHECK-NEXT:    vsetvli zero, zero, e16, m4, ta, ma
-; CHECK-NEXT:    vadd.vv v12, v16, v12
+; CHECK-NEXT:    vwmacc.vx v8, a0, v16
+; CHECK-NEXT:    vwmacc.vx v12, a1, v16
 ; CHECK-NEXT:    vsetvli zero, a4, e8, m4, ta, ma
 ; CHECK-NEXT:    vse8.v v8, (a2)
 ; CHECK-NEXT:    vse8.v v12, (a3)