[DAGCombine] Don't delete the node if it has uses immediately

This is the follow up patch for https://reviews.llvm.org/D86183 as we miss to delete the node if NegX == NegY, which has use after we create the node.
```
    if (NegX && (CostX <= CostY)) {
      Cost = std::min(CostX, CostZ);
      RemoveDeadNode(NegY);
      return DAG.getNode(Opcode, DL, VT, NegX, Y, NegZ, Flags);  #<-- NegY is used here if NegY == NegX.
    }
```

Reviewed By: spatel

Differential Revision: https://reviews.llvm.org/D86689

Author: QingShan Zhang

llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp

@@ -5820,15 +5820,17 @@ SDValue TargetLowering::getNegatedExpression(SDValue Op, SelectionDAG &DAG,
     // Negate the X if its cost is less or equal than Y.
     if (NegX && (CostX <= CostY)) {
       Cost = CostX;
+      SDValue N = DAG.getNode(ISD::FSUB, DL, VT, NegX, Y, Flags);
       RemoveDeadNode(NegY);
-      return DAG.getNode(ISD::FSUB, DL, VT, NegX, Y, Flags);
+      return N;
     }
 
     // Negate the Y if it is not expensive.
     if (NegY) {
       Cost = CostY;
+      SDValue N = DAG.getNode(ISD::FSUB, DL, VT, NegY, X, Flags);
       RemoveDeadNode(NegX);
-      return DAG.getNode(ISD::FSUB, DL, VT, NegY, X, Flags);
+      return N;
     }
     break;
   }
@@ -5865,8 +5867,9 @@ SDValue TargetLowering::getNegatedExpression(SDValue Op, SelectionDAG &DAG,
     // Negate the X if its cost is less or equal than Y.
     if (NegX && (CostX <= CostY)) {
       Cost = CostX;
+      SDValue N = DAG.getNode(Opcode, DL, VT, NegX, Y, Flags);
       RemoveDeadNode(NegY);
-      return DAG.getNode(Opcode, DL, VT, NegX, Y, Flags);
+      return N;
     }
 
     // Ignore X * 2.0 because that is expected to be canonicalized to X + X.
@@ -5877,8 +5880,9 @@ SDValue TargetLowering::getNegatedExpression(SDValue Op, SelectionDAG &DAG,
     // Negate the Y if it is not expensive.
     if (NegY) {
       Cost = CostY;
+      SDValue N = DAG.getNode(Opcode, DL, VT, X, NegY, Flags);
       RemoveDeadNode(NegX);
-      return DAG.getNode(Opcode, DL, VT, X, NegY, Flags);
+      return N;
     }
     break;
   }
@@ -5907,15 +5911,17 @@ SDValue TargetLowering::getNegatedExpression(SDValue Op, SelectionDAG &DAG,
     // Negate the X if its cost is less or equal than Y.
     if (NegX && (CostX <= CostY)) {
       Cost = std::min(CostX, CostZ);
+      SDValue N = DAG.getNode(Opcode, DL, VT, NegX, Y, NegZ, Flags);
       RemoveDeadNode(NegY);
-      return DAG.getNode(Opcode, DL, VT, NegX, Y, NegZ, Flags);
+      return N;
     }
 
     // Negate the Y if it is not expensive.
     if (NegY) {
       Cost = std::min(CostY, CostZ);
+      SDValue N = DAG.getNode(Opcode, DL, VT, X, NegY, NegZ, Flags);
       RemoveDeadNode(NegX);
-      return DAG.getNode(Opcode, DL, VT, X, NegY, NegZ, Flags);
+      return N;
     }
     break;
   }

llvm/test/CodeGen/PowerPC/fneg.ll

@@ -39,3 +39,20 @@ define float @fma_fneg_fsub(float %x, float %y0, float %y1, float %z) {
   %r = call float @llvm.fmuladd.f32(float %negx, float %negy, float %z)
   ret float %r
 }
+
+; Verify that we didn't hit assertion for this case.
+define double @fneg_no_ice(float %x) {
+; CHECK-LABEL: fneg_no_ice:
+; CHECK:       # %bb.0:
+; CHECK-NEXT:    lis r3, .LCPI3_0@ha
+; CHECK-NEXT:    lfs f0, .LCPI3_0@l(r3)
+; CHECK-NEXT:    fsubs f0, f0, f1
+; CHECK-NEXT:    fmul f1, f0, f0
+; CHECK-NEXT:    fmul f1, f0, f1
+; CHECK-NEXT:    blr
+  %y = fsub fast float 1.0, %x
+  %e = fpext float %y to double
+  %e2 = fmul double %e, %e
+  %e3 = fmul double %e, %e2
+  ret double %e3
+}