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

[segher at gcc dot gnu.org]

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

--- Comment #3 from Segher Boessenkool <segher at gcc dot gnu.org> ---
It should do something like

  if (!VECTOR_UNIT_NONE_P (V2DImode))
    return V2DImode;

and similar for all existing entries.

Putting the same conditionals in multiple places is prone to error, as
this PR shows.