Add regression test for #6655

Fixed by 2b52368 in #10014

Author: griggt

tests/pos/i6655.scala

@@ -0,0 +1,10 @@
+class Test {
+  def foo(): Unit = {
+    val x: String|Null = ???
+    if (x != null) {
+      val y = x.length
+    } else {
+      val y = x
+    }
+  }
+}