Translates a mlir::BlockArgument to an mlir::Value more directly
representing the entity or expression to which the block argument
refers, or null if not applicable for the statement in question.
`mlir::Type` is now used as the default C++ type for representing all
MLIR types, even when `ValueType` would be a valid constraint, unless
doing so provides a clear benefit in that specific context.
Lauri Vasama