isl_ast_codegen.c: set_for_node_expressions: drop redundant isl_ast_build_copy
isl_ast_codegen.c: set_for_node_expressions: drop redundant isl_ast_build_copy

This was missing from isl-0.13-107-gd708098a1f (drop
isl_ast_build_set_enforced, Mon Jul 28 17:38:17 2014 +0200).

Signed-off-by: Sven Verdoolaege <dd860110a62b19214c4ee03aec0abffecb4e86b8@cerebras.net>

Meinersbur/isl,Meinersbur/isl,Meinersbur/isl,Meinersbur/isl