[XLA] Add dead tuple elem removal to WhileLoopSimplifier.

Specifically, if a while loop has tuple element that

 - is not used by the while condition, and
 - is not used by the while body, except to pass it along to the next
   iteration of the loop,

then we can reshape the while loop's computations to eliminate this
tuple element.

PiperOrigin-RevId: 174413683