[llvm] [RISCVGatherScatterLowering] Support vp_gather and vp_scatter (PR #73612)

[Craig Topper via llvm-commits]

topperc wrote:

> It seems Roger favor a new patch. https://discourse.llvm.org/t/rfc-improved-vplan-native-path/73152/4 When the patch landing, we will have vp_gather/vp_scatter to trigger this patch.

We had something like this patch in our downstream, but we got a lot better results doing it in the vectorizer with SCEV information like what's in the BSC vectorizer.

https://github.com/llvm/llvm-project/pull/73612