Show existing entries in Switch

[jyn514]

• What is the feature or code improvement you would like to do in Cranelift?

I would like Switch to have a new method entries() added which returns an immutable list of the existing entries.

• What is the value of adding this in Cranelift?

This would allow me to check in the codegen module whether a case expression has already been added, instead of having to frontload it in the parser (which for various reasons would be possible but annoying).

• Do you have an implementation plan, and/or ideas for data structures or algorithms to use?

fn entries(&self) -> &HashMap<EntryIndex, Ebb> {
    &self.cases
}

• Have you considered alternative implementations? If so, how are they better or worse than your proposal?

Alternatively, Cranelift could impl Deref<Target = HashMap<...>> for Switch. This would also provide more methods at the same time.

Cranelift could also choose to do nothing at all.

[jyn514]

Added in https://github.com/bytecodealliance/cranelift/pull/1358