Move the `getMutableSuccessorOperands` implementation from `ReturnLike`
trait to the `RegionBranchTerminatorOpInterface` to allow overriding of
the implementation. This allows to have the trait on operations that are
not a return of all of their operands. This can be used, for example, to
implement custom `ReturnLike` terminator that consumes non-returned
operands in combination with `func.func`.
The `RegionBranchTerminatorOpInterface` now provides a default
implementation for the `getMutableSuccessorOperands` method that returns
all of the operands.
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