[Bug target/104976] [avx512fp16] lowpart_subreg return NULL_RTX cause ICE

[crazylht at gmail dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104976

--- Comment #4 from Hongtao.liu <crazylht at gmail dot com> ---
ICE is fixed in GCC12, and I'd like to keep this PR open for refining
validate_subreg in GCC13.