[llvm] [GlobalISel] Add support for most G_VECREDUCE_* operations to moreElementsVector (PR #81830)

[Dhruv Chawla via llvm-commits]

dc03-work wrote:

> Hi - this sounds really good. It's nice to start getting support for these operations.
> 
> We usually prefer that patches are committed alongside tests, so I think it would make sense to combine this in with #81831. That way we can add the AArch64 legalization at the same time and make sure it's properly tested.

The problem with merging with #81831 is that it only focuses on adding support for SMIN and friends, so it would still not end up testing the whole function. TBH I'm not sure how to test these functions, because that would require finding cases for all these operations where we need to add more elements to the vector.

For AArch64, one solution could be to limit this patch to just SMIN and friends for now, but I feel that's not very beneficial to other backends who could use the support for all operations.

https://github.com/llvm/llvm-project/pull/81830