[Bug target/121358] [15/16 Regression] SVE ICE compiling highway since r15-4235

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=121358

--- Comment #4 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
So I think the problem is force_subreg expected simplify_gen_subreg would
simplify the subreg of the const_vector into a const_int but in this case we
don't as we don't have a way to simplify const_vectors with const_poly_int.