[X86][Costmodel] `trunc v8i64 to v16i8/v32i8/v64i8` can appear after...

[X86][Costmodel] `trunc v8i64 to v16i8/v32i8/v64i8` can appear after legalization, cost is same as for `trunc v8i64 to v8i8`

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/D113851