skip tests/codegen/swap-small-types when debug assertions are on

In `swap_nonoverlapping_short` there's a new `debug_assert!`, and if that's enabled then the `alloca`s don't optimize out.

Author: scottmcm

Diff for: tests/codegen/swap-small-types.rs

@@ -1,6 +1,7 @@
 //@ compile-flags: -Copt-level=3 -Z merge-functions=disabled
 //@ only-x86_64
 //@ min-llvm-version: 20
+//@ ignore-std-debug-assertions (`ptr::swap_nonoverlapping` has one which blocks some optimizations)
 
 #![crate_type = "lib"]