[LiveIntervalUnion] Expose extraction of last index in map for external users

Differential Revision: https://reviews.llvm.org/D69515

llvm/include/llvm/CodeGen/LiveIntervalUnion.h

@@ -75,6 +75,7 @@ public:
 
   bool empty() const { return Segments.empty(); }
   SlotIndex startIndex() const { return Segments.start(); }
+  SlotIndex endIndex() const { return Segments.stop(); }
 
   // Provide public access to the underlying map to allow overlap iteration.
   using Map = LiveSegments;