skip_stages doesn't respect debug_to_file, leading to runtime crashes.

[abadams]

The use of debug_to_file does not force a stage to be allocated, even though it was a use of the buffer, so you get a segfault at runtime when the debug_to_file implementation reads a bad pointer. Repro:

#include "Halide.h"
using namespace Halide;

int main() {
    Var x, y;
    Func f, g, h;

    Param<bool> p;
    f(x, y) = x + y;
    g(x, y) = f(x, y);
    h(x, y) = select(p, g(x, y), 0);

    f.compute_root().debug_to_file("/dev/null");
    g.compute_root();

    p.set(false);

    h.realize({1024, 1024});

    printf("Success!\n");

    return 0;
}

This is in fact already fixed by the new skip stages implementation in #8115, but I figured I'd open (and immediately close) the bug in case someone using Halide earlier than main is searching for it, to make them aware that it's fixed on main.