Don't build a ty::Const just to take it apart again

compiler/rustc_mir/src/interpret/intrinsics.rs

@@ -171,8 +171,7 @@ pub fn emulate_intrinsic(
                 };
                 let val =
                     self.tcx.const_eval_global_id(self.param_env, gid, Some(self.tcx.span))?;
-                let const_ = ty::Const { val: ty::ConstKind::Value(val), ty };
-                let val = self.const_to_op(&const_, None)?;
+                let val = self.const_val_to_op(val, ty, None)?;
                 self.copy_op(&val, dest)?;
             }