compound assigns desugar to plain assignment + binary; increment
and decrement (statement or expression position) keep their own
ir node and emit `++` / `--` directly.

<<<

main = fun () {
    var x int = 0
    x += 1
    x *= 2
    x++
    x -= 3
    x--
    var y int = x++
    var z int = ++x
}

>>>

#include "core.c"
void catskill_main(void);
void catskill_main(void) {
    integer x = 0;
    x = x + 1;
    x = x * 2;
    x++;
    x = x - 3;
    x--;
    integer y = x++;
    integer z = ++x;
}
#include "runtime.c"