Codegen has two separate, slightly different functions for finding admissible conditional inames

The functions get_usable_inames_for_conditional (loopy/codegen/bounds.py) and get_admissible_conditional_inames_for (loopy/codegen/control.py) seem at first glance like they ought be unified, because they have the same goal of finding a set of inames which may appear in conditionals. If not, then there should be documentation which explains under which circumstances either function should be used.