[DebugInfo][InstrRef][NFC] Bypass a frequently-noop loop

[jmorse]

Summary

Bypass a loop if it would do nothing -- if there are no register masks to be examined, there's no point looking at each location to see if the location has been def'd. Awkwardly, this was responsible for almost an entire half a percent of performance improvement on the tracker [0].

[0] http://llvm-compile-time-tracker.com/compare.php?from=47a80eaac7a4fb1b115a5fb467ce03b51182fc56&to=19a73bdd29dcc0f522aba1e722f7a3a16cbea4dd&stat=instructions

[StephenTozer]

LGTM, that's a depressingly good performance gain/prevented cost.

llvm/lib/CodeGen/LiveDebugValues/InstrRefBasedImpl.cpp

void InstrRefBasedLDV::transferRegisterDef(MachineInstr &MI) {
   // Inform TTracker about any direct clobbers.
   for (uint32_t DeadReg : DeadRegs) {
     LocIdx Loc = MTracker->lookupOrTrackRegister(DeadReg);
     TTracker->clobberMloc(Loc, MI.getIterator(), false);
   }
 
   // Look for any clobbers performed by a register mask. Only test locations
   // that are actually being tracked.
+  if (!RegMaskPtrs.empty()) {
     for (auto L : MTracker->locations()) {
       // Stack locations can't be clobbered by regmasks.
       if (MTracker->isSpill(L.Idx))
         continue;
 
       Register Reg = MTracker->LocIdxToLocID[L.Idx];
       if (IgnoreSPAlias(Reg))
         continue;
 
       for (auto *MO : RegMaskPtrs)
         if (MO->clobbersPhysReg(Reg))
           TTracker->clobberMloc(L.Idx, MI.getIterator(), false);
     }
+  }
 
   // Tell TTracker about any folded stack store.
   if (hasFoldedStackStore(MI)) {
     if (Optional<SpillLocationNo> SpillNo = extractSpillBaseRegAndOffset(MI)) {
       for (unsigned int I = 0; I < MTracker->NumSlotIdxes; ++I) {
         unsigned SpillID = MTracker->getSpillIDWithIdx(*SpillNo, I);
         LocIdx L = MTracker->getSpillMLoc(SpillID);
         TTracker->clobberMloc(L, MI.getIterator(), true);