Add AddDependency HLO instruction.

The AddDependency HLO instruction takes a data operand and a token operand. The output is the data operand. When used with AfterAll (which has been extended in the CL to accept non-token operands), this enables ordering of instructions which do produce or accept tokens (ie, non-side-effecting ops). This new functionality is intentionally not exposed in the XlaBuilder interface because the ordering of non-side-effecting operations is fragile to some optimizations. Rather this new op will be used to replace existing uses of control dependencies which are to be removed. The uses of these new ordering ops will be late in the optimization pipeline and will avoid the more problematic optimization interactions.

See go/xla-side-effect-semantics for further details.

PiperOrigin-RevId: 222445994