Add a regression test for SR-8469 / rdar://43888895

Author: slavapestov

validation-test/compiler_crashers_2_fixed/sr8469.swift

@@ -0,0 +1,21 @@
+// RUN: %target-swift-frontend -emit-ir %s
+
+final class Baz {}
+
+final class Bar {
+    private let x: Baz
+    init(x: Baz) {
+        self.x = x
+    }
+}
+
+final class Foo {
+    private var bar: Bar?
+
+    private func navigate(with baz: Baz?) {
+        bar = nil
+        guard let baz = baz else { return }
+        let bar = Bar(x: baz)
+        self.bar = bar
+    }
+}