[Bug target/82735] _mm256_zeroupper does not invalidate previously computed registers

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82735

--- Comment #4 from Richard Biener <rguenth at gcc dot gnu.org> ---
Indeed as far as I understand an unspec volatile isn't sth clobbering
registers (not even memory?!).  The insn is missing inputs/outputs
(we might be able to model that lowparts are preserved).