The check for race conditions doesn't consider where clauses

[abadams]

Halide should require allow_race_conditions() to be willing to compile the following code, but it doesn't. The where clause introduces a race.

    Func f;
    Var x;
    RDom r(0, 10);
    f(x) = 1;
    r.where(f(0) == 1);
    f(r) = 2;

    f.update().parallel(r);

ATTN @alexreinking