Auto merge of #87916 - nbdd0121:black_box, r=nagisa

Implement `black_box` using intrinsic

Introduce `black_box` intrinsic, as suggested in rust-lang/rust#87590 (comment).

This is still codegenned as empty inline assembly for LLVM. For MIR interpretation and cranelift it's treated as identity.

cc `@Amanieu` as this is related to inline assembly
cc `@bjorn3` for rustc_codegen_cranelift changes
cc `@RalfJung` as this affects MIRI

r? `@nagisa` I suppose

Author: bors

src/intrinsics/mod.rs

@@ -1136,6 +1136,11 @@ pub(crate) fn codegen_intrinsic_call<'tcx>(
                 };
             ret.write_cvalue(fx, CValue::by_val(is_eq_value, ret.layout()));
         };
+
+        black_box, (c a) {
+            // FIXME implement black_box semantics
+            ret.write_cvalue(fx, a);
+        };
     }
 
     if let Some((_, dest)) = destination {