emit an assume that cast-from enums are in range

Fixes #36955.

src/test/codegen/enum-bounds-check.rs

@@ -0,0 +1,24 @@
+// Copyright 2016 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// compile-flags: -O
+
+#![crate_type = "lib"]
+
+pub enum Foo {
+    A, B
+}
+
+// CHECK-LABEL: @lookup
+#[no_mangle]
+pub fn lookup(buf: &[u8; 2], f: Foo) -> u8 {
+    // CHECK-NOT: panic_bounds_check
+    buf[f as usize]
+}