Prevent miscompilation in trivial loop {}

Ideally, we would want to handle a broader set of cases to fully fix the
underlying bug here. That is currently relatively expensive at compile and
runtime, so we don't do that for now.

src/test/codegen/loop.rs

@@ -0,0 +1,15 @@
+// compile-flags: -C opt-level=3
+
+#![crate_type = "lib"]
+
+// CHECK-LABEL: @check_loop
+#[no_mangle]
+pub fn check_loop() -> u8 {
+    // CHECK-NOT: unreachable
+    call_looper()
+}
+
+#[no_mangle]
+fn call_looper() -> ! {
+    loop {}
+}