[SLP] Add ScalarizationOverheadBuilder helper to track vector extractions

Instead of accumulating all extraction costs separately and then adjusting for repeated subvector extractions, this patch collects all the extractions and then converts to calls to getScalarizationOverhead to improve the accuracy of the costs.

I'm not entirely satisfied with the getExtractWithExtendCost handling yet - this still just adds all the getExtractWithExtendCost costs together - it really needs to be replaced with a "getScalarizationOverheadWithExtend", but that will require further refactoring first.

This replaces my initial attempt in D124769.

Differential Revision: https://reviews.llvm.org/D134605