This is a general midend pass for any target that does not support action_run directly. It rewrites the actions in a table that uses action_run to set a newly introduced metadata tmp enum that records which action was run, and changes the switch statement to use that.

It is kind-of logically the reverse of EliminateSwitch, which converts switch statements that don't use action_run() into ones that do, so it would never make sense to use both passes for a backend.

Need to update p4test so we can select which pass(es) to run in a test by some annotation in the test program to better allow testing this.