[llvm] X86: make VBMI2 funnel shifts use VSHLD/VSHRD for const splats (PR #169401)

[Simon Pilgrim via llvm-commits]

https://github.com/RKSimon requested changes to this pull request.

Now that its legal, much of the VBMI2 path can be removed from LowerFunnelShift - we just need to perform the widening for non-VLX VBMI2 targets.

https://github.com/llvm/llvm-project/pull/169401