Add regression test for fixed crasher

Author: slavapestov

validation-test/compiler_crashers_2_fixed/sr9584.swift

@@ -0,0 +1,14 @@
+// RUN: not %target-swift-frontend -typecheck %s
+
+struct S<N> {}
+
+protocol P {
+  associatedtype A: P = Self
+  static func f(_ x: A) -> A
+}
+
+extension S: P where N: P {
+  static func f<X: P>(_ x: X) -> S<X.A> where A == X, X.A == N {
+    return S<X.A>()
+  }
+}