Teach LiveIntervals how to verify themselves and start using it in some

of the trick merge routines. This adds a layer of testing that was
necessary when implementing more efficient (and complex) merge logic for
this datastructure.

No functionality changed here.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@159981 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/CodeGen/LiveInterval.h

@@ -489,6 +489,15 @@ namespace llvm {
     void print(raw_ostream &OS) const;
     void dump() const;
 
+    /// \brief Walk the interval and assert if any invariants fail to hold.
+    ///
+    /// Note that this is a no-op when asserts are disabled.
+#ifdef NDEBUG
+    void verify() const {}
+#else
+    void verify() const;
+#endif
+
   private:
 
     Ranges::iterator addRangeFrom(LiveRange LR, Ranges::iterator From);