[VectorCombine] foldConcatOfBoolMasks - 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

@@ -1532,6 +1532,10 @@ bool VectorCombine::foldConcatOfBoolMasks(Instruction &I) {
   if (ShAmtX > 0)
     NewCost += TTI.getArithmeticInstrCost(Instruction::Shl, Ty, CostKind);
 
+  LLVM_DEBUG(dbgs() << "Found a concatenation of bitcasted bool masks: " << I
+                    << "\n  OldCost: " << OldCost << " vs NewCost: " << NewCost
+                    << "\n");
+
   if (NewCost > OldCost)
     return false;