Support RngBitGenerator HloInstruction with single output in the HLO -> MHLO conversion.
Support RngBitGenerator HloInstruction with single output in the HLO -> MHLO conversion.

For the rng-bit-generator operator, xla HLO instruction can have two kinds of shapes, (1) tuple(output_state, output_data), and (2) output_data. On the contrary, `mhlo::RngBitGeneratorOp` has only one shape, (output_state, output_data). This cl supports RngBitGenerator HloInstruction with single output in the HLO -> MHLO conversion.
