cgwidentaggedstorebp's struct-ident arms (local and module-global)
counted copy bytes from structinfo.totsize, the round-8 slot width, so
a struct whose natural size is not a multiple of 8 took the MOVQ word
loop and copied the source slot's dirty pad bytes over the zero-fill
the arm itself just established; the width-true tail emitter beneath
was dead code. cstage keys the same copy on su->size. Route both arms
through copysrcnatsize (the stamped-tinfo natural size, #71).
Graduates r71_tagged_return_scratch_mixed.
Hojun-Cho
c717facf78