[Reduce] Argument reduction: do deal with function declarations

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.