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

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