Rollup merge of #149087 - nxsaken:unchecked_neg_shifts_stabilize, r=Amanieu

Stabilize `unchecked_neg` and `unchecked_shifts`

Features: `unchecked_neg`, `unchecked_shifts`
Tracking issue: rust-lang/rust#85122

r? `@Amanieu`

Author: Zalathar

tests/fail/intrinsics/unchecked_shl.rs

@@ -1,5 +1,3 @@
-#![feature(unchecked_shifts)]
-
 fn main() {
     unsafe {
         let _n = 1i8.unchecked_shl(8);

tests/fail/intrinsics/unchecked_shr.rs

@@ -1,5 +1,3 @@
-#![feature(unchecked_shifts)]
-
 fn main() {
     unsafe {
         let _n = 1i64.unchecked_shr(64);