[X86][Costmodel] `trunc v16i32 to v32i16` can appear after legalization, cost...

[X86][Costmodel] `trunc v16i32 to v32i16` can appear after legalization, cost is same as for `trunc v16i32 to v16i16`

This was noticed in D113609, hopefully it unblocks that patch.
There are likely other similar problems.

Reviewed By: RKSimon

Differential Revision: https://reviews.llvm.org/D113842