[VectorCombine] foldInsExtVectorToShuffle - 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

@@ -3003,6 +3003,10 @@ bool VectorCombine::foldInsExtVectorToShuffle(Instruction &I) {
   if (!Ext->hasOneUse())
     NewCost += TTI.getVectorInstrCost(*Ext, VecTy, CostKind, ExtIdx);
 
+  LLVM_DEBUG(dbgs() << "Found a insert/extract shuffle-like pair : " << I
+                    << "\n  OldCost: " << OldCost << " vs NewCost: " << NewCost
+                    << "\n");
+
   if (OldCost < NewCost)
     return false;