[VPlan] Replace LLVM_ATTRIBUTE_USED with ifndef NDEBUG

[dcaballe]

Summary

Minor replacement. LLVM_ATTRIBUTE_USED was introduced to silence a warning but using #ifndef NDEBUG makes more sense in this case.

[dblaikie]

Looks good - thanks!

lib/Transforms/Vectorize/VPlanVerifier.cpp

 #define DEBUG_TYPE "loop-vectorize"
 
 using namespace llvm;
 
 static cl::opt<bool> EnableHCFGVerifier("vplan-verify-hcfg", cl::init(false),
                                         cl::Hidden,
                                         cl::desc("Verify VPlan H-CFG."));
 
+#ifndef NDEBUG
 /// Utility function that checks whether \p VPBlockVec has duplicate
 /// VPBlockBases.
-LLVM_ATTRIBUTE_USED static bool
-hasDuplicates(const SmallVectorImpl<VPBlockBase *> &VPBlockVec) {
+static bool hasDuplicates(const SmallVectorImpl<VPBlockBase *> &VPBlockVec) {
   SmallDenseSet<const VPBlockBase *, 8> VPBlockSet;
   for (const auto *Block : VPBlockVec) {
     if (VPBlockSet.count(Block))
       return true;
     VPBlockSet.insert(Block);
   }
   return false;
 }
+#endif
 
 /// Helper function that verifies the CFG invariants of the VPBlockBases within
 /// \p Region. Checks in this function are generic for VPBlockBases. They are
 /// not specific for VPBasicBlocks or VPRegionBlocks.
 static void verifyBlocksInRegion(const VPRegionBlock *Region) {
   for (const VPBlockBase *VPB :
        make_range(df_iterator<const VPBlockBase *>::begin(Region->getEntry()),
                   df_iterator<const VPBlockBase *>::end(Region->getExit()))) {