[llvm] [SDAG] Add partial_reduce_sumla node (PR #141267)

[Philip Reames via llvm-commits]

preames wrote:

I've landed tests in main, and merged them into the dev branch.  I've gone through and done some cleanup/generalization based on the additional tests.  This should be ready for regularly review.  

https://github.com/llvm/llvm-project/pull/141267