[X86][Costmodel] `trunc v16i32 to v32i8/v64i8` can appear after legalization,...

[X86][Costmodel] `trunc v16i32 to v32i8/v64i8` can appear after legalization, cost is same as for `trunc v16i32 to v16i8`

While this one is trivial and identical to the previous patch,
there is a weird cost change in a follow-up patch that i'm not sure about.

Reviewed By: RKSimon

Differential Revision: https://reviews.llvm.org/D113850