Don't ICE on operator trait methods with generic methods

Emit a fatal error instead.

src/test/ui/traits/invalid_operator_trait.rs

@@ -0,0 +1,23 @@
+#![crate_type = "lib"]
+#![feature(lang_items)]
+#![feature(no_core)]
+#![no_core]
+
+#[lang="sized"]
+pub trait Sized {
+    // Empty.
+}
+
+#[lang = "add"]
+trait Add<RHS=Self> {
+    type Output;
+
+    fn add<Y>(self, _: RHS) -> Self::Output;
+    //~^ ERROR `add` must not have any generic parameters
+}
+
+#[allow(unreachable_code)]
+fn ice(a: usize) {
+    let r = loop {};
+    r = r + a;
+}

src/test/ui/traits/invalid_operator_trait.stderr

@@ -0,0 +1,8 @@
+error: `add` must not have any generic parameters
+  --> $DIR/invalid_operator_trait.rs:15:5
+   |
+LL |     fn add<Y>(self, _: RHS) -> Self::Output;
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to previous error
+