[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 #5 from Segher Boessenkool <segher at gcc dot gnu.org> ---
The whole function can be something as simple as

  mode = mode_for_vector (mode, 16 / GET_MODE_SIZE (mode));
  if (this is actually an existing mode
      && !VECTOR_UNIT_NONE (mode))
    return mode;

  return word_mode;

Or, why not?