isl_ast_node_print: avoid duplicate declarations in printed code

isl_ast_node_print: avoid duplicate declarations in printed code

If a loop has only one iteration but if the expression
for this single iteration is too complicated to be substituted into the AST,
then the expression is assigned to a variable corresponding to the loop
iterator.
If two such loops occur in the same block, then we would end up with
two declarations of the same variable.  Avoid this problem by introducing
a block around the declaration if it appears in a sequence of statements.

Reported-by: Tobias Grosser <tobias@grosser.es>
Signed-off-by: Sven Verdoolaege <skimo@kotnet.org>