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

[Philip Reames via llvm-commits]

preames wrote:

#141970 has landed, and I've merged main into the dev branch.  Still need to improve test coverage of the new node, but this should leave WIP-status in a day or so.  

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