Add a test written by jdm that tests repeated vector self appending.

src/test/run-pass/vec-self-append.rs

@@ -0,0 +1,12 @@
+// xfail-stage0
+
+fn main()
+{
+    // Make sure we properly handle repeated self-appends.
+    let vec[int] a = [0];
+    auto i = 20;
+    while (i > 0) {
+        a += a;
+        i -= 1;
+    }
+}