[inductor] use triu ref instead of lowering (#96040) (#96462)
Fixes #95958
Generated code is functionally identical with ref and lowering, only minor differences
Pull Request resolved: https://github.com/pytorch/pytorch/pull/96040
Approved by: https://github.com/jansel
Co-authored-by: Natalia Gimelshein <ngimel@fb.com>
