[CostModel][X86] getScalarizationOverhead - improve extraction costs for > 128-bit vectors

We were using the default getScalarizationOverhead expansion for extraction costs, which adds up all the individual element extraction costs.

This is fine for 128-bit vectors, but for 256/512-bit vectors each element extraction also has to account for extracting the upper 128-bit subvector extraction before it can handle the element. For scalarization costs we only need to extract each demanded subvector once.

Differential Revision: https://reviews.llvm.org/D125527