If you don't see the practical advantage of being able to determine
whether an insn is part of a libcall without having to walk the entire
function to figure out whether you are allowed to touch an insn,
please just remove it.
I've posted a patch to that effect. It restores the correct interpretation of
libcall notes as optimization helpers rather than optimization barriers as
far as DCE is concerned and, thus, eliminates the need for being able to
determine whether an insn is part of a libcall in dce.c.
But I won't install it myself, given how controversial this issue has become.
I therefore propose that a 3rd party decides here, and I'd suggest Jakub.