Generator: derive property swap from node.

Generator: derive property swap from node.

Previously, the "in" operator swap flag was relayed through generator
context.  This broke when call-end handlers switched to
njs_generator_stack_pop(NULL), which released the context before the
swap was read in njs_generate_3addr_operation_end().

The fix derives the swap directly from the node token type
(NJS_TOKEN_IN), eliminating the context relay.