Add function to extract outside compilation in XLA computations.

Unit test encapsulate_subgraphs_pass_test.cc now uses this function to extract outside compilation.

Two small changes w.r.t. original implementation:
1. Now we can infer output shapes in more cases (because we do shape inference earlier);
2. If there is control dependency between nodes in different outside compilations, do not add control dependency between their XlaHostCompute nodes.

PiperOrigin-RevId: 219211543