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

[ezhulenev]

Fix bugs

[ezhulenev]

Rebase

llvm/lib/CodeGen/LiveDebugValues/InstrRefBasedImpl.cpp

auto isCalleeSaved = [&](LocIdx L) {
       if (Reg >= MTracker->NumRegs)
         return false;
       for (MCRegAliasIterator RAI(Reg, &TRI, true); RAI.isValid(); ++RAI)
         if (CalleeSavedRegs.test(*RAI))
           return true;
       return false;
     };
 
+    // Short-circuit unnecessary preferred location map construction.
+    if (VLocs.empty()) {
+      flushDbgValues(MBB.begin(), &MBB);
+      return;
+    }
+
     // Map of the preferred location for each value.
     std::map<ValueIDNum, LocIdx> ValueToLoc;
     ActiveMLocs.reserve(VLocs.size());
     ActiveVLocs.reserve(VLocs.size());
 
     // 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.