CodeGen: fix child nodes usage

[ehaas]

This was caused by #750 ; not sure if there's an easy way to add a test at the moment. I tested manually with

extern int printf(const char *restrict fmt, ...);
int main(void) {
    int x = ({
        printf("Hello, world!\n");
        2;
    });
    return 0;
}

on x86_64 linux.