[NFC][SLP] Improved description of getShallowScore() and getScoreAtLevelRec()

[vdmitrie]

Thank you Vasileios. Looks good.

llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp

class VLOperands {
     /// Matching with an undef is preferable to failing.
     static const int ScoreUndef = 1;
     /// Score for failing to find a decent match.
     static const int ScoreFail = 0;
     /// Score if all users are vectorized.
     static const int ScoreAllUserVectorized = 1;
 
     /// \returns the score of placing \p V1 and \p V2 in consecutive lanes.
+    /// \p U1 and \p U2 are the users of \p V1 and \p V2.
     /// Also, checks if \p V1 and \p V2 are compatible with instructions in \p
     /// MainAltOps.
     int getShallowScore(Value *V1, Value *V2, Instruction *U1, Instruction *U2,
                         const DataLayout &DL, ScalarEvolution &SE, int NumLanes,
                         ArrayRef<Value *> MainAltOps) {
       if (V1 == V2) {
         if (isa<LoadInst>(V1)) {
           // Retruns true if the users of V1 and V2 won't need to be extracted.
int getExternalUseScore(unsigned Lane, unsigned OpIdx, unsigned Idx) const {
       if (!IdxLaneI || !isa<Instruction>(OpIdxLaneV))
         return 0;
       return R.areAllUsersVectorized(IdxLaneI, None)
                  ? VLOperands::ScoreAllUserVectorized
                  : 0;
     }
 
     /// Go through the operands of \p LHS and \p RHS recursively until \p
-    /// MaxLevel, and return the cummulative score. For example:
+    /// MaxLevel, and return the cummulative score. \p U1 and \p U2 are
+    /// the users of \p LHS and \p RHS (that is \p LHS and \p RHS are operands
+    /// of \p U1 and \p U2), except at the beginning of the recursion where
+    /// these are set to nullptr.
+    ///
+    /// For example:
     /// \verbatim
     ///  A[0]  B[0]  A[1]  B[1]  C[0] D[0]  B[1] A[1]
     ///     \ /         \ /         \ /        \ /
     ///      +           +           +          +
     ///     G1          G2          G3         G4
     /// \endverbatim
     /// The getScoreAtLevelRec(G1, G2) function will try to match the nodes at
     /// each level recursively, accumulating the score. It starts from matching