[IPO] Avoid repeated hash lookups (NFC) (#129467)

Author: kazutakahirata

llvm/lib/Transforms/IPO/FunctionAttrs.cpp

@@ -319,8 +319,9 @@ static FunctionSummary *calculatePrevailingSummary(
     function_ref<bool(GlobalValue::GUID, const GlobalValueSummary *)>
         IsPrevailing) {
 
-  if (CachedPrevailingSummary.count(VI))
-    return CachedPrevailingSummary[VI];
+  if (auto It = CachedPrevailingSummary.find(VI);
+      It != CachedPrevailingSummary.end())
+    return It->second;
 
   /// At this point, prevailing symbols have been resolved. The following leads
   /// to returning a conservative result: