[NFC][IntrRefLDV] Remove dead code from transferSpillOrRestoreInst()

[djtodoro]

Summary

This is an NFC.

[jmorse]

LGTM; the point of the "Emulate VarLocBasedLDV" flag was to help prove that the overall algorithm matched what was already produced by LiveDebugValues. Seeing how the approach seems to be (broadly?) accepted / acceptable, it's to just ditch the flag altogether, it doesn't serve any purpose now IMO.

[djtodoro]

OK, IMPO we can go with removing it completely from the source.

llvm/lib/CodeGen/LiveDebugValues/InstrRefBasedImpl.cpp

if (isLocationSpill(MI, MF, Reg)) {
     auto OptSpillLocIdx = MTracker->getSpillMLoc(*Loc);
     assert(OptSpillLocIdx && "Spill slot set but has no LocIdx?");
     LocIdx SpillLocIdx = *OptSpillLocIdx;
 
     // Tell TransferTracker about this spill, produce DBG_VALUEs for it.
     if (TTracker)
       TTracker->transferMlocs(MTracker->getRegMLoc(Reg), SpillLocIdx,
                               MI.getIterator());
-
-    // VarLocBasedImpl would, at this point, stop tracking the source
-    // register of the store.
-    if (EmulateOldLDV) {
-      for (MCRegAliasIterator RAI(Reg, TRI, true); RAI.isValid(); ++RAI)
-        MTracker->defReg(*RAI, CurBB, CurInst);
-    }
   } else {
     if (!(Loc = isRestoreInstruction(MI, MF, Reg)))
       return false;
 
     // Is there a value to be restored?
     auto OptValueID = MTracker->readSpill(*Loc);
     if (OptValueID) {
       ValueIDNum ValueID = *OptValueID;