LiveIntervalAnalysis: add an assertion in repairOldRegInRange

[nhaehnle]

Summary

This is triggered by a test case I have, which otherwise leads to
non-deterministic crashes making it rather harder to even
bugpoint-reduce...

[nhaehnle]

Please see D23303 instead.

lib/CodeGen/LiveIntervalAnalysis.cpp

void LiveIntervals::repairOldRegInRange(const MachineBasicBlock::iterator Begin,
                                         const MachineBasicBlock::iterator End,
                                         const SlotIndex endIdx,
                                         LiveRange &LR, const unsigned Reg,
                                         LaneBitmask LaneMask) {
   LiveInterval::iterator LII = LR.find(endIdx);
   SlotIndex lastUseIdx;
   if (LII != LR.end() && LII->start < endIdx)
     lastUseIdx = LII->end;
-  else
+  else {
+    assert(LII != LR.begin());
     --LII;
+  }
 
   for (MachineBasicBlock::iterator I = End; I != Begin;) {
     --I;
     MachineInstr &MI = *I;
     if (MI.isDebugValue())
       continue;
 
     SlotIndex instrIdx = getInstructionIndex(MI);