[VectorCombine] foldBitcastShuffle - add debug message for match + cost-comparison

Helps with debugging to show to that the fold found the match, and shows the old + new costs to indicate whether the fold was/wasn't profitable.

Author: RKSimon

llvm/lib/Transforms/Vectorize/VectorCombine.cpp

@@ -794,17 +794,21 @@ bool VectorCombine::foldBitcastShuffle(Instruction &I) {
       IsUnary ? TargetTransformInfo::SK_PermuteSingleSrc
               : TargetTransformInfo::SK_PermuteTwoSrc;
 
-  InstructionCost DestCost =
+  InstructionCost NewCost =
       TTI.getShuffleCost(SK, NewShuffleTy, NewMask, CostKind) +
       (NumOps * TTI.getCastInstrCost(Instruction::BitCast, NewShuffleTy, SrcTy,
                                      TargetTransformInfo::CastContextHint::None,
                                      CostKind));
-  InstructionCost SrcCost =
+  InstructionCost OldCost =
       TTI.getShuffleCost(SK, SrcTy, Mask, CostKind) +
       TTI.getCastInstrCost(Instruction::BitCast, DestTy, OldShuffleTy,
                            TargetTransformInfo::CastContextHint::None,
                            CostKind);
-  if (DestCost > SrcCost || !DestCost.isValid())
+
+  LLVM_DEBUG(dbgs() << "Found a bitcasted shuffle: " << I << "\n  OldCost: "
+                    << OldCost << " vs NewCost: " << NewCost << "\n");
+
+  if (NewCost > OldCost || !NewCost.isValid())
     return false;
 
   // bitcast (shuf V0, V1, MaskC) --> shuf (bitcast V0), (bitcast V1), MaskC'