[X86][Costmodel] `trunc v32i16 to v64i8` can appear after legalization, cost...

[X86][Costmodel] `trunc v32i16 to v64i8` can appear after legalization, cost is same as for `trunc v32i16 to v32i8`

Some of the costs get larger here,
but i suppose that makes sense since we'd previously query
scalarization costs that may not be really representative of the reality.

Reviewed By: RKSimon

Differential Revision: https://reviews.llvm.org/D113852