Splitter fixes

[robknight]

Fixes two issues with the predicate splitter:

• In cases where public arguments are not used further, drop them from the remaining predicates in the chain
• When deciding how to weight statements, model the use of private arguments as more expensive than public arguments, to minimise unnecessary predicate-crossing

There are some other minor re-organisations of code, including a fix to an error message and the removal of some unused "wildcard analysis" code.

[ed255]

LGTM! Thanks for fixing this!