[VectorCombine] Add a debug message for foldShuffleOfCastop. NFC

This optimization, much like the existing foldShuffleOfBinops can cause a
lot of regressions. Add a quick debug message to make the costs are more
obvious.

Author: davemgreen

llvm/lib/Transforms/Vectorize/VectorCombine.cpp

@@ -1485,6 +1485,10 @@ bool VectorCombine::foldShuffleOfCastops(Instruction &I) {
       TargetTransformInfo::SK_PermuteTwoSrc, CastSrcTy, Mask, CostKind);
   NewCost += TTI.getCastInstrCost(Opcode, ShuffleDstTy, NewShuffleDstTy,
                                   TTI::CastContextHint::None, CostKind);
+
+  LLVM_DEBUG(dbgs() << "Found a shuffle feeding two casts: " << I
+                    << "\n  OldCost: " << OldCost << " vs NewCost: " << NewCost
+                    << "\n");
   if (NewCost > OldCost)
     return false;