[VectorCombine] Add "VC: Erasing" debug message to help the log show when dead WorkList instructions are erased.

Author: RKSimon

llvm/lib/Transforms/Vectorize/VectorCombine.cpp

@@ -138,6 +138,7 @@ class VectorCombine {
   }
 
   void eraseInstruction(Instruction &I) {
+    LLVM_DEBUG(dbgs() << "VC: Erasing: " << I << '\n');
     for (Value *Op : I.operands())
       Worklist.pushValue(Op);
     Worklist.remove(&I);