[XLA] Don't use NumUniqueInstructionIds() as a proxy for instruction_count()

It used to be a reasonable proxy, but that's no longer the case. This is because GetUniqueId() in XlaBuilder uses a *global* (rather than a module-global) counter. Since HloModule::CreateFromProto no-longer uniquifies ids coming in from protos, the potentially very high IDs coming from GetUniqueId() become the module's next_unique_id.

There is another case of this in TuplePointsTo, that will be handled separately.

PiperOrigin-RevId: 214614576