For MIRI, cfg out the swap vectorization logic from 94212 Because of #69488 the swap logic from #94212 doesn't currently work in MIRI. Copying in smaller pieces is probably much worse for its performance anyway, so it'd probably rather just use the simple path regardless. Part of #94371, though another PR will be needed for the CTFE aspect. r? `@oli-obk` cc `@RalfJung`