Ahaggar: mark more *_EXPR nodes as expr statements

Signed-off-by: Djalal Harouni <tixxdz@opendz.org>
Ahaggar: fix a bug in the EXIT_EXPR node walker

* This is a copy/past typo. In the EXIT_EXPR walker we need to pass the
  address of the TREE_OPERAND(node, 0) node

Signed-off-by: Djalal Harouni <tixxdz@opendz.org>