Auto merge of #51171 - faern:const-bswap-ctpop-cttz-ctlz, r=oli-obk

Make some std::intrinsics `const fn`s

Making some rustc intrinsics (`ctpop`, `cttz`, `ctlz` and `bswap`) `const fn`s.

This is a pre-step to being able to make `swap_bytes`, `to_be` and `from_be` constant functions. That in itself could be ergonomic and useful. But even better is that it would allow `Ipv4Addr::new` etc becoming `const fn`s as well. Which might be really useful since I find it quite common to want to define them as constants.

r? @oli-obk

src/test/run-pass/ctfe/bswap-const.rs

@@ -0,0 +1,23 @@
+// Copyright 2017 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.
+
+#![feature(core_intrinsics)]
+
+use std::intrinsics;
+
+const SWAPPED_U8: u8 = unsafe { intrinsics::bswap(0x12_u8) };
+const SWAPPED_U16: u16 = unsafe { intrinsics::bswap(0x12_34_u16) };
+const SWAPPED_I32: i32 = unsafe { intrinsics::bswap(0x12_34_56_78_i32) };
+
+fn main() {
+    assert_eq!(SWAPPED_U8, 0x12);
+    assert_eq!(SWAPPED_U16, 0x34_12);
+    assert_eq!(SWAPPED_I32, 0x78_56_34_12);
+}