[llvm] [X86][GISEL] Lower Vector shuffle (PR #201055)

[Simon Pilgrim via llvm-commits]

RKSimon wrote:

It'd be much better if we could get buildvector, extract/insert element/subvector handled properly first - have we finalised how vector indices are supposed to be handled yet?

https://github.com/llvm/llvm-project/pull/201055