We can happily turn function definitions into declarations,
thus obscuring their argument from being elided by this pass.
I don't believe there is a good reason to just ignore declarations.
likely even proper llvm intrinsics ones,
at worst the input becomes uninteresting.
The other question here is that all these transforms are all-or-nothing.
In some cases, should we be treating each use separately?
The main blocker here seemed to be that llvm::CloneFunctionInto()
does `&OldFunc->front()`, which inserts a nullptr into a densemap,
which is not happy about it and asserts.
As it can be seen in newly-added (previously-crashing) test-case,
there can be a situation where multiple arguments are used in instr,
and we would schedule the same instruction to be deleted several times,
crashing when trying to delete it the second time.
We could either store WeakVH (done here), or use something set-like.
I think using WeakVH is prevalent in these cases elsewhere.
Roman Lebedev