Enable constant propagation across Switch(x,x) by rewriting the two outputs as...

Enable constant propagation across Switch(x,x) by rewriting the two outputs as Const(false), Const(true) with appropriate control dependencies. This is a fairly common pattern when the graph contains assertions.

By rewriting the graph a bit, we can propagate the constants down the two output branches, and just use control dependencies to trigger the selected one at runtime. For example,
         +------+
     x-->|Switch|-->a
     x-->|      |-->b
         +------+

Is rewritten as

         +------+
     x-->|Switch|-->Identity--^>Const(false)-->a
     x-->|      |-->Identity--^>Const(true)-->b
         +------+

(In practice there may be multiple consumers of each output branch.)

PiperOrigin-RevId: 186714991