[Bug target/79202] On Power8, consider using vupkhsw/xxpermdi to sign extend an int in a vector register instead of mfvsrwz/mtvsrwa

[bergner at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=79202

--- Comment #3 from Peter Bergner <bergner at gcc dot gnu.org> ---
(In reply to Peter Bergner from comment #2)
> Can we instead do:
> 
>         xscvdpsxws 32,1
>         vupkhsw 1,0
>         xvcvsxwdp 3,33

Replying to myself, the answer is no, the vupkhsw doesn't write its results in
the correct words for xvcvsxwdp.