Rollup merge of #121894 - RalfJung:const_eval_select, r=oli-obk

const_eval_select: make it safe but be careful with what we expose on stable for now

As this is all still nightly-only I think `````@rust-lang/wg-const-eval````` can do that without involving t-lang.

r? `````@oli-obk`````
Cc `````@Nilstrieb````` -- the updated version of your RFC would basically say that we can remove these comments about not making behavior differences visible in stable `const fn`

compiler/rustc_hir_analysis/src/check/intrinsic.rs

@@ -132,7 +132,8 @@ pub fn intrinsic_operation_unsafety(tcx: TyCtxt<'_>, intrinsic_id: LocalDefId) -
         | sym::fsub_algebraic
         | sym::fmul_algebraic
         | sym::fdiv_algebraic
-        | sym::frem_algebraic => hir::Unsafety::Normal,
+        | sym::frem_algebraic
+        | sym::const_eval_select => hir::Unsafety::Normal,
         _ => hir::Unsafety::Unsafe,
     };