Strategy: there is now a regular flow step from an instance-of LHS / switch expr to the pattern, 0 or more read steps corresponding to record pattern destructors, and then finally a normal SSA def/use step connecting the binding patterns to their first uses.
These were reasonably broken beforehand, due to not taking switch rules into account in enough places, and confusing the expression/statement switch rule distinction with the distinction between switch statements and expressions.
(For example, `switch(x) { 1 -> System.out.println("Hello world") ... }` is a statement, but has a rule expression).
