[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 #2 from Hongtao.liu <crazylht at gmail dot com> ---
(In reply to Hongtao.liu from comment #1)
> The walkround in the backend is force_reg operand[1] before lowpart_subreg
> to avoid NULL_RTX.

It would be nice if we extend validate_subreg to avoid wired situation which
allows (subreg:V8HF (reg:V2HF ) 0) and (subreg:V2HF (reg:SI) 0) but not
(subreg:V8HF (reg:SI) 0)