Auto merge of #122582 - scottmcm:swap-intrinsic-v2, r=oli-obk

Let codegen decide when to `mem::swap` with immediates

Making `libcore` decide this is silly; the backend has so much better information about when it's a good idea.

Thus this PR introduces a new `typed_swap` intrinsic with a fallback body, and replaces that fallback implementation when swapping immediates or scalar pairs.

r? oli-obk

Replaces #111744, and means we'll never need more libs PRs like #111803 or #107140

compiler/rustc_hir_analysis/src/check/intrinsic.rs

@@ -484,6 +484,8 @@ pub fn check_intrinsic_type(
                 (1, 0, vec![Ty::new_mut_ptr(tcx, param(0)), param(0)], Ty::new_unit(tcx))
             }
 
+            sym::typed_swap => (1, 1, vec![Ty::new_mut_ptr(tcx, param(0)); 2], Ty::new_unit(tcx)),
+
             sym::discriminant_value => {
                 let assoc_items = tcx.associated_item_def_ids(
                     tcx.require_lang_item(hir::LangItem::DiscriminantKind, None),