[DAGCombiner] improve readability

This is not quite NFC because I changed the SDLoc to use the more
standard 'N' (the starting node for the fold).

This transform is a special-case of a more general fold that we
do in IR, but it seems like the general fold is needed here too
to avoid a potential regression seen in D58017.

https://rise4fun.com/Alive/3jZm