make devirt func pass work for functions with different calling conv.
make devirt func pass work for functions with different calling conv.

By courtesy of danblitzhou

ashish-gehani/OCCAM,ashish-gehani/OCCAM,SRI-CSL/OCCAM,ashish-gehani/OCCAM,SRI-CSL/OCCAM,SRI-CSL/OCCAM,SRI-CSL/OCCAM,ashish-gehani/OCCAM,SRI-CSL/OCCAM