[MachineOutliner] Assert that Liveness tracking is accurate (NFC)

[yroux]

Summary

The checking is done deeper inside MachineBasicBlock, but this will hopefully help to find issues when porting the machine outliner to a target where Liveness tracking is broken (like ARM) .

[paquette]

include/llvm/CodeGen/MachineOutliner.h

145

Can you add a message to the assert like "Candidate's MF must track liveness"? It's a bit nicer to have some context with asserts in my experience.

[yroux]

include/llvm/CodeGen/MachineOutliner.h

145

Yes I hesitated to add one, but you're right it'd be more helpfull

[paquette]

LGTM

include/llvm/CodeGen/MachineOutliner.h

bool operator<(const Candidate &RHS) const {
     return getStartIdx() > RHS.getStartIdx();
   }
 
   /// Compute the registers that are live across this Candidate.
   /// Used by targets that need this information for cost model calculation.
   /// If a target does not need this information, then this should not be
   /// called.
   void initLRU(const TargetRegisterInfo &TRI) {
+    assert(MBB->getParent()->getRegInfo().tracksLiveness());
     LRU.init(TRI);
     LRU.addLiveOuts(*MBB);
 
     // Compute liveness from the end of the block up to the beginning of the
     // outlining candidate.
     std::for_each(MBB->rbegin(), (MachineBasicBlock::reverse_iterator)front(),
                   [this](MachineInstr &MI) { LRU.stepBackward(MI); });
   }

lib/Target/AArch64/AArch64InstrInfo.cpp

AArch64InstrInfo::getMachineOutlinerMBBFlags(MachineBasicBlock &MBB) const {
   // Check if there's a call inside this MachineBasicBlock. If there is, then
   // set a flag.
   if (std::any_of(MBB.begin(), MBB.end(),
                   [](MachineInstr &MI) { return MI.isCall(); }))
     Flags |= MachineOutlinerMBBFlags::HasCalls;
 
   // Check if LR is available through all of the MBB. If it's not, then set
   // a flag.
+  assert(MBB->getParent()->getRegInfo().tracksLiveness());
   LiveRegUnits LRU(getRegisterInfo());
   LRU.addLiveOuts(MBB);
 
   std::for_each(MBB.rbegin(),
                 MBB.rend(),
                 [&LRU](MachineInstr &MI) { LRU.accumulate(MI); });
 
   if (!LRU.available(AArch64::LR))