[Bug target/92566] rs6000_preferred_simd_mode isn't very good

[linkw at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=92566

--- Comment #6 from Kewen Lin <linkw at gcc dot gnu.org> ---
Great! I was thinking there whether exists some array to map from mode to
vector, but missed this one. Good to know we have this kind of function!