Add a forgotten initializer for CheckedFirstInterference.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@121410 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/CodeGen/LiveIntervalUnion.cpp b/lib/CodeGen/LiveIntervalUnion.cpp
index d5c7bf0..59d48a4 100644
--- a/lib/CodeGen/LiveIntervalUnion.cpp
+++ b/lib/CodeGen/LiveIntervalUnion.cpp
@@ -168,6 +168,8 @@
       IR.VirtRegI = VirtReg->end();
   }
   findIntersection(FirstInterference);
+  assert((IR.VirtRegI == VirtReg->end() || IR.LiveUnionI.valid())
+         && "Uninitialized iterator");
   return FirstInterference;
 }