[NFC] Fix the formula for reciprocal calculation.

[LuoYuanke]

llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp

22880

The original formula (F(X) = A X - 1) is mis-leading. It doesn't fit "X_{i+1} = X_i - F(X_i)/F'(X_i)"

llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp

SDValue DAGCombiner::BuildLogBase2(SDValue V, const SDLoc &DL) {
   SDValue Ctlz = DAG.getNode(ISD::CTLZ, DL, VT, V);
   SDValue Base = DAG.getConstant(VT.getScalarSizeInBits() - 1, DL, VT);
   SDValue LogBase2 = DAG.getNode(ISD::SUB, DL, VT, Base, Ctlz);
   return LogBase2;
 }
 
 /// Newton iteration for a function: F(X) is X_{i+1} = X_i - F(X_i)/F'(X_i)
 /// For the reciprocal, we need to find the zero of the function:
-///   F(X) = A X - 1 [which has a zero at X = 1/A]
+///   F(X) = 1/X - A [which has a zero at X = 1/A]
 ///     =>
 ///   X_{i+1} = X_i (2 - A X_i) = X_i + X_i (1 - A X_i) [this second form
 ///     does not require additional intermediate precision]
 /// For the last iteration, put numerator N into it to gain more precision:
 ///   Result = N X_i + X_i (N - N A X_i)
 SDValue DAGCombiner::BuildDivEstimate(SDValue N, SDValue Op,
                                       SDNodeFlags Flags) {
   if (LegalDAG)