Replace `discriminant_switch_effect` with more general version
#68528 added a new edge-specific effect for `SwitchInt` terminators, `discriminant_switch_effect`, to the dataflow framework. While this accomplished the short-term goal of making drop elaboration more precise, it wasn't really useful in other contexts: It only supported `SwitchInt`s on the discriminant of an `enum` and did not allow effects to be applied along the "otherwise" branch. In const-propagation, for example, arbitrary edge-specific effects for the targets of a `SwitchInt` can be used to remember the value a `match` scrutinee must have in each arm.
This PR replaces `discriminant_switch_effect` with a more general `switch_int_edge_effects` method. The new method has a slightly different interface from the other edge-specific effect methods (e.g. `call_return_effect`). This divergence is explained in the new method's documentation, and reading the changes to the various dataflow impls as well as `direction.rs` should further clarify things. This PR should not change behavior.
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