[LiveIntervalAnalysis] Don't dereference an end iterator in repairIntervalsInRange

This fixes a bug introduced in:

  r262115 - CodeGen: Take MachineInstr& in SlotIndexes and LiveIntervals, NFC

The iterator End here might == MBB->end(), and so we can't unconditionally
dereference it. This often goes unnoticed (I don't have a test case that always
crashes, and ASAN does not catch it either) because the function call arguments are
turned right back into iterators. MachineInstrBundleIterator's constructor,
however, does have an assert which might randomly fire.

llvm-svn: 270323