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`