[RegisterScavenger] Fix handling of predicated instructions

[tobiasvk]

Summary

The RegisterScavenger explicitly ignores <kill> flags on operands of
predicated instructions and therefore assumes that such registers remain
live. When it then scavenges such a register, it inserts a spill of this
(killed) register. This is invalid code and gets flagged up by the
verifier.

Nowadays kill flags are set correctly on predicated instructions. This
patch makes the Scavenger respect them.

The bug has so far only been triggered by an internal pass, so I don't
have a test case unfortunately.

Fixes PR23119.

[tobiasvk]

Rebased

lib/CodeGen/RegisterScavenging.cpp

 void RegScavenger::determineKillsAndDefs() {
   assert(Tracking && "Must be tracking to determine kills and defs");
 
   MachineInstr *MI = MBBI;
   assert(!MI->isDebugValue() && "Debug values have no kills or defs");
 
   // Find out which registers are early clobbered, killed, defined, and marked
   // def-dead in this instruction.
-  // FIXME: The scavenger is not predication aware. If the instruction is
-  // predicated, conservatively assume "kill" markers do not actually kill the
-  // register. Similarly ignores "dead" markers.
-  bool isPred = TII->isPredicated(MI);
   KillRegUnits.reset();
   DefRegUnits.reset();
   for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {
     const MachineOperand &MO = MI->getOperand(i);
     if (MO.isRegMask()) {
       
       TmpRegUnits.clear();
       for (unsigned RU = 0, RUEnd = TRI->getNumRegUnits(); RU != RUEnd; ++RU) {
         for (MCRegUnitRootIterator RURI(RU, TRI); RURI.isValid(); ++RURI) {
           if (MO.clobbersPhysReg(*RURI)) {
             TmpRegUnits.set(RU);
             break;
           }
         }
       }
       
       // Apply the mask.
-      (isPred ? DefRegUnits : KillRegUnits) |= TmpRegUnits;
+      KillRegUnits |= TmpRegUnits;
     }
     if (!MO.isReg())
       continue;
     unsigned Reg = MO.getReg();
     if (!Reg || TargetRegisterInfo::isVirtualRegister(Reg) || isReserved(Reg))
       continue;
 
     if (MO.isUse()) {
       // Ignore undef uses.
       if (MO.isUndef())
         continue;
-      if (!isPred && MO.isKill())
+      if (MO.isKill())
         addRegUnits(KillRegUnits, Reg);
     } else {
       assert(MO.isDef());
-      if (!isPred && MO.isDead())
+      if (MO.isDead())
         addRegUnits(KillRegUnits, Reg);
       else
         addRegUnits(DefRegUnits, Reg);
     }
   }
 }
 
 void RegScavenger::unprocess() {