[llvm] [X86][CostModel] Cost gathers by the instructions CodeGen emits (PR #220565)

[Sumukh J Bharadwaj via llvm-commits]

amd-subharad wrote:

> Please take a look at the PR summary and the release notes - neither are easier to comprehend, ideally they should be shorter and more direct.
> 
> I fear something that is going to bite us is trying to get this to work for -Os/-Oz AVX2 builds where we want to use gather to save on codesize - see #224229

Hello @RKSimon, Sorry about the unnecessarily long comments and description, I will simplify it and address your other queries as well along with the new coments from @MattPD 
Thank you

https://github.com/llvm/llvm-project/pull/220565