The tests/verilog/*_if_enc.ys scripts instantiate simple encoder
modules, both with and without the SystemVerilog priority/unique/unique0
keywords, and check for consistency between the two for the subset
of inputs where the priority/unique/unique0 "if" result is
well-defined.
These tests vacuously succeed at the moment, since priority/unique
keywords are silently ignored and therefore the generated logic is
trivially identical. But the test cases will be capable of detecting
certain types of unsound optimisation if priority/unique handling is
introduced later.
