[SLP][NFC]Limit number of the external uses analysis, NFC.

BoUpSLP::buildExternalUses runs through all the users of the vectorized
scalars, which may require significant amount of time, if there are too
many users. Limited the analysis, if there are too many users, all of
them are replaced, not individually.

Author: alexey-bataev

llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp

@@ -5892,6 +5892,8 @@ void BoUpSLP::buildExternalUses(
           }
         }
 
+        if (U && Scalar->hasNUsesOrMore(UsesLimit))
+          U = nullptr;
         int FoundLane = Entry->findLaneForValue(Scalar);
         LLVM_DEBUG(dbgs() << "SLP: Need to extract:" << *UserInst
                           << " from lane " << FoundLane << " from " << *Scalar
@@ -13940,6 +13942,7 @@ Value *BoUpSLP::vectorizeTree(
       if (!ScalarsWithNullptrUser.insert(Scalar).second)
         continue;
       assert((ExternallyUsedValues.count(Scalar) ||
+              Scalar->hasNUsesOrMore(UsesLimit) ||
               any_of(Scalar->users(),
                      [&](llvm::User *U) {
                        if (ExternalUsesAsGEPs.contains(U))