[ThinLTO] Simplify checking for single external copy (NFCI) (#164861)

Replace a loop over all summary copies with a simple check for a single
externally available copy of a symbol. The usage of this result has
changed since it was added and we now only need to know if there is a
single one.

Author: teresajohnson

llvm/lib/LTO/LTO.cpp

@@ -471,16 +471,14 @@ static void thinLTOInternalizeAndPromoteGUID(
     ValueInfo VI, function_ref<bool(StringRef, ValueInfo)> isExported,
     function_ref<bool(GlobalValue::GUID, const GlobalValueSummary *)>
         isPrevailing) {
-  auto ExternallyVisibleCopies =
-      llvm::count_if(VI.getSummaryList(),
-                     [](const std::unique_ptr<GlobalValueSummary> &Summary) {
-                       return !GlobalValue::isLocalLinkage(Summary->linkage());
-                     });
-
   // Before performing index-based internalization and promotion for this GUID,
   // the local flag should be consistent with the summary list linkage types.
   VI.verifyLocal();
 
+  const bool SingleExternallyVisibleCopy =
+      VI.getSummaryList().size() == 1 &&
+      !GlobalValue::isLocalLinkage(VI.getSummaryList().front()->linkage());
+
   for (auto &S : VI.getSummaryList()) {
     // First see if we need to promote an internal value because it is not
     // exported.
@@ -543,7 +541,9 @@ static void thinLTOInternalizeAndPromoteGUID(
         GlobalValue::isExternalWeakLinkage(S->linkage()))
       continue;
 
-    if (isPrevailing(VI.getGUID(), S.get()) && ExternallyVisibleCopies == 1)
+    // We may have a single summary copy that is externally visible but not
+    // prevailing if the prevailing copy is in a native object.
+    if (SingleExternallyVisibleCopy && isPrevailing(VI.getGUID(), S.get()))
       S->setLinkage(GlobalValue::InternalLinkage);
   }
 }