[LiveDebugValues] Avoid repeated hash lookups (NFC)

Author: kazutakahirata

llvm/lib/CodeGen/LiveDebugValues/InstrRefBasedImpl.cpp

@@ -2231,11 +2231,9 @@ void InstrRefBasedLDV::accumulateFragmentMap(MachineInstr &MI) {
   // If this is the first sighting of this variable, then we are guaranteed
   // there are currently no overlapping fragments either. Initialize the set
   // of seen fragments, record no overlaps for the current one, and return.
-  auto SeenIt = SeenFragments.find(MIVar.getVariable());
-  if (SeenIt == SeenFragments.end()) {
-    SmallSet<FragmentInfo, 4> OneFragment;
-    OneFragment.insert(ThisFragment);
-    SeenFragments.insert({MIVar.getVariable(), OneFragment});
+  auto [SeenIt, Inserted] = SeenFragments.try_emplace(MIVar.getVariable());
+  if (Inserted) {
+    SeenIt->second.insert(ThisFragment);
 
     OverlapFragments.insert({{MIVar.getVariable(), ThisFragment}, {}});
     return;