add pet_stmt_can_build_ast_exprs and pet_scop_can_build_ast_exprs
pet_stmt_build_ast_exprs is currently limited to only handle
the kinds of data dependent accesses that we are currently able
to parse. In subsequent commits, we will be extending the types of data
dependent accesses we can parse, while the extension of
pet_stmt_build_ast_exprs will be postponed to a later date
such that pet_stmt_build_ast_exprs may fail in some cases.
These functions allow the user to check in advance whether
pet_stmt_build_ast_exprs will succeed.
Signed-off-by: Sven Verdoolaege <skimo@kotnet.org>