feat: add control flow branch interface

[axelcool1234]

Implemented a very simple interface for branch operations. I skipped a lot of extra stuff (such as supporting branch ops that produce operands internally by the branch op - something VeIR has none of right now) that's in here: https://github.com/llvm/llvm-project/blob/main/mlir/include/mlir/Interfaces/ControlFlowInterfaces.h but it's a start. This will help #854 which would use this interface. The reason SuccessorOperands is a one field structure is to later have additional fields like the number of produced operands like I mentioned. Right now it'd be unused so I left it as is.

[math-fehr]

Nice! Can you move getSuccessorOperands to HasOpInfo though? And just return an Array ValuePtr?

[regehr]

looks nice overall but I definitely agree that the dialect-specific parts of this need to be pushed into the dialect files

[regehr]

also use a WfIRContext instead of a raw context?

[axelcool1234]

also use a WfIRContext instead of a raw context?

None of the other interfaces I've looked at does this

looks nice overall but I definitely agree that the dialect-specific parts of this need to be pushed into the dialect files

Will do, I'll move things around

[math-fehr]

Okay nice, I just have some comments on the notation then (since you are removing the checks that are not necessary). And then that should be good!

[math-fehr]

Nice! Sorry I just have one last request, but I'll merge it right after!

[math-fehr]

Thanks!