Implement unary chain hoisting optimization for Concat, Split, and SplitV.

For Concat, hoist prefix chains of unary ops before concatenation, e.g.
  // Rewrites
  //       Concat({Cos(Exp(a)), Cos(Exp(b)), Cos(Exp(c))})
  // into
  //       Cos(Exp(Concat({a, b, c}))).

For Split/SplitV hoist unary postfix chains before the split, e.g.
  // Rewrites
  //          [Cos(Exp(y)) for y in Split(x)]
  // into
  //          [y for y in Split(Cos(Exp(x)))].

The new optimization is off by default.

PiperOrigin-RevId: 194850318