[X86][Costmodel] `trunc v8i64 to v16i16/v32i16` can appear after legalization,...

[X86][Costmodel] `trunc v8i64 to v16i16/v32i16` can appear after legalization, cost is same as for `trunc v8i64 to v8i16`

Same as D113842, but for i64

Reviewed By: RKSimon

Differential Revision: https://reviews.llvm.org/D113843