[DebugInfo][InstrRef] Short-circuit unnecessary preferred location map construction

[ezhulenev]

Fix bugs

[ezhulenev]

Rebase

llvm/lib/CodeGen/LiveDebugValues/InstrRefBasedImpl.cpp

void loadInlocs(MachineBasicBlock &MBB, ValueIDNum *MLocs,
 
     // Produce a map of value numbers to the current machine locs they live
     // in. When emulating VarLocBasedImpl, there should only be one
     // location; when not, we get to pick.
     for (auto Location : MTracker->locations()) {
       LocIdx Idx = Location.Idx;
       ValueIDNum &VNum = MLocs[Idx.asU64()];
       VarLocs.push_back(VNum);
+
+      // Short-circuit unnecessary preferred location update.
+      if (VLocs.empty())
+        continue;
+
       auto it = ValueToLoc.find(VNum);
       // In order of preference, pick:
       //  * Callee saved registers,
       //  * Other registers,
       //  * Spill slots.
       if (it == ValueToLoc.end() || MTracker->isSpill(it->second) ||
           (!isCalleeSaved(it->second) && isCalleeSaved(Idx.asU64()))) {
         // Insert, or overwrite if insertion failed.