[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 #8 from Segher Boessenkool <segher at gcc dot gnu.org> ---
I don't think you need lines 4909..4911.

How can we test this?  Is there good test coverage for it already?