Add assert for valid slot indexes.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@115649 91177308-0d34-0410-b5e6-96231b3b80d8

lib/CodeGen/LiveInterval.cpp

@@ -40,6 +40,7 @@ struct CompEnd {
 }
 
 LiveInterval::iterator LiveInterval::find(SlotIndex Pos) {
+  assert(Pos.isValid() && "Cannot search for an invalid index");
   return std::upper_bound(begin(), end(), LiveRange(SlotIndex(), Pos, 0),
                           CompEnd());
 }