[IR] Avoid repeated hash lookups (NFC) (#110450)

Author: kazutakahirata

llvm/lib/IR/AsmWriter.cpp

@@ -1338,12 +1338,8 @@ void SlotTracker::CreateMetadataSlot(const MDNode *N) {
 void SlotTracker::CreateAttributeSetSlot(AttributeSet AS) {
   assert(AS.hasAttributes() && "Doesn't need a slot!");
 
-  as_iterator I = asMap.find(AS);
-  if (I != asMap.end())
-    return;
-
-  unsigned DestSlot = asNext++;
-  asMap[AS] = DestSlot;
+  if (asMap.try_emplace(AS, asNext).second)
+    ++asNext;
 }
 
 /// Create a new slot for the specified Module