RegAllocGreedy: Fix nondeterminism in tryLastChanceRecoloring

[arsenm]

Summary

tryLastChanceRecoloring iterates over the set of LiveInterval pointers
and used that to seed the recoloring stack, which was
nondeterministic. Fixes a future test failing about 20% of the time.

This just takes the order the interfering vreg was encountered. Not
sure if we should try to order this more intelligently.

[qcolombet]

Good catch!

[arsenm]

bfdca1535c0d6b8ace3de69ec2e448d93a88b97f

llvm/lib/CodeGen/RegAllocGreedy.h

public:
   const RegisterClassInfo &getRegClassInfo() const { return RegClassInfo; }
   const ExtraRegInfo &getExtraInfo() const { return *ExtraInfo; }
   size_t getQueueSize() const { return Queue.size(); }
   // end (interface to eviction advisers)
 
 private:
   // Convenient shortcuts.
   using PQueue = std::priority_queue<std::pair<unsigned, unsigned>>;
-  using SmallLISet = SmallPtrSet<const LiveInterval *, 4>;
+  using SmallLISet = SmallSetVector<const LiveInterval *, 4>;
 
   // We need to track all tentative recolorings so we can roll back any
   // successful and unsuccessful recoloring attempts.
   using RecoloringStack =
       SmallVector<std::pair<const LiveInterval *, MCRegister>, 8>;
 
   // context
   MachineFunction *MF;