[llvm] [VPlan] Make widen-gather-scatter decision in VPlan (PR #215862)

[Ramkumar Ramachandra via llvm-commits]

artagnon wrote:

More work is necessary, as tests show. I just put this up early to motivate https://github.com/llvm/llvm-project/pull/215857.

https://github.com/llvm/llvm-project/pull/215862