[X86][Costmodel] `*ext v64i1 to v32i16` can appear after legalization, cost is...

[X86][Costmodel] `*ext v64i1 to v32i16` can appear after legalization, cost is same as for `*ext v32i1 to v32i16`

Reviewed By: RKSimon

Differential Revision: https://reviews.llvm.org/D113914