[SLP]Skip undefs trying to find perfect/shuffled tree entries matching.

[ABataev]

Summary

We can skip check for undefs trying to find perfect/shuffled tree
entries matching, they can be ignored completely improving the final
cost/vectorization results.

[RKSimon]

LGTM

llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp

BoUpSLP::isGatherShuffledEntry(const TreeEntry *TE, SmallVectorImpl<int> &Mask,
   assert(VLIt != VectorizableTree.end() &&
          "Gathered values should be in the tree.");
   Mask.assign(TE->Scalars.size(), UndefMaskElem);
   Entries.clear();
   DenseMap<const TreeEntry *, int> Used;
   int NumShuffles = 0;
   for (int I = 0, E = TE->Scalars.size(); I < E; ++I) {
     Value *V = TE->Scalars[I];
+    if (isa<UndefValue>(V))
+      continue;
     const TreeEntry *VTE = getTreeEntry(V);
     if (!VTE) {
       // Check if it is used in one of the gathered entries.
       const auto *It =
           find_if(make_range(VectorizableTree.begin(), VLIt),
                   [V](const std::unique_ptr<TreeEntry> &EntryPtr) {
                     return EntryPtr->State == TreeEntry::NeedToGather &&
                            is_contained(EntryPtr->Scalars, V);

llvm/test/Transforms/SLPVectorizer/X86/remark_extract_broadcast.ll

 ; CHECK-NEXT:    store <8 x i16> [[TMP10]], <8 x i16>* [[TMP11]], align 2
 ; CHECK-NEXT:    ret void
 ;
 ; YAML:      Pass:            slp-vectorizer
 ; YAML-NEXT: Name:            StoresVectorized
 ; YAML-NEXT: Function:        fextr
 ; YAML-NEXT: Args:
 ; YAML-NEXT:   - String:          'Stores SLP vectorized with cost '
-; YAML-NEXT:   - Cost:            '1'
+; YAML-NEXT:   - Cost:            '-1'
 ; YAML-NEXT:   - String:          ' and with tree size '
 ; YAML-NEXT:   - TreeSize:        '4'
 
 entry:
   %LD = load <8 x i16>, <8 x i16>* undef
   %V0 = extractelement <8 x i16> %LD, i32 0
   br label %t