PATCH: PR rtl-optimization/50696: [x32] Unnecessary lea

[Richard Kenner]

> The answer to H.J.'s "Why do we do it for MEM then?" is simply
> "because no one ever thought about not doing it" 

No, that's false.  The same expand_compound_operation / make_compound_operation
pair is present in the MEM case as in the SET case.  It's just that
there's some bug here that's noticable in not making proper MEMs that
doesn't show up in the SET case because of the way the insns are structured.