UseListOrder: Handle self-users

Correctly sort self-users (such as PHI nodes).  I added a targeted test
in `test/Bitcode/use-list-order.ll` and the final missing RUN line to
tests in `test/Assembly`.

This is part of PR5680.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@214417 91177308-0d34-0410-b5e6-96231b3b80d8

test/Bitcode/use-list-order.ll

@@ -118,3 +118,16 @@ entry:
   %local = load i4* @globalAndFunction
   ret i4 %local
 }
+
+; Check for when an instruction is its own user.
+define void @selfUser() {
+entry:
+  ret void
+
+loop1:
+  br label %loop2
+
+loop2:
+  %var = phi i32 [ %var, %loop1 ], [ %var, %loop2 ]
+  br label %loop1
+}