[Bug tree-optimization/124545] VN nary lookup trick for converted plus/minus lacks inverse trick

[ptomsich at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=124545

--- Comment #5 from ptomsich at gcc dot gnu.org ---
I had completely forgotten about this one, but just found the branch and will
move this forward.