Add an assertion to make PR7542 fail consistently.

LiveInterval::overlapsFrom dereferences end() if it is called on an empty
interval.

It would be reasonable to just return false - an empty interval doesn't overlap
anything, but I want to know who is doing it first.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@108264 91177308-0d34-0410-b5e6-96231b3b80d8

lib/CodeGen/LiveInterval.cpp

@@ -119,6 +119,7 @@ bool LiveInterval::killedInRange(SlotIndex Start, SlotIndex End) const {
 //
 bool LiveInterval::overlapsFrom(const LiveInterval& other,
                                 const_iterator StartPos) const {
+  assert(!empty() && "empty interval");
   const_iterator i = begin();
   const_iterator ie = end();
   const_iterator j = StartPos;