A belt needs to have congruence maintained wherever two or more control flows join. The includes call and return sites, and also branch-like control flow when a value dropped in one ebb is passed to and consumed by ops in a different ebb. The argument lists of call and return provide a ready-made way to enforce congruence: the arguments must be in the same order at each site, which is easy to enforce.
The erstwhile “conforms” op rearranged the belt for branch join points. It was a separate op so that it could be omitted if by happenstance (or good scheduling) the live values were already congruent on the in-bound control flow arcs. It turns out that such natural congruence almost never happens, so we replaced the conforms op with a call-like argument list on the branches.
You suggestion to re-order call arguments to get them into belt lifetime order is valid, although the current specializer does not do it. In fact, it does no partial-parameterization yet at all – not just ordering, but such things as folding common constant arguments into the body of the called functions. Some of that sort of thing are done by LLVM’s Link Time Optimization facility, and so will also be done for Mill targets. However, argument reordering is not done by LLVM, and we have no plans to add it to LLVM any time soon.