Add HloSchedule class representing a sequential order of an HloModule.

Currently we represent a sequential schedule of a module using a  SequentialHloOrdering::HloModuleSequence which is a type alias of a bare map from HloComputation* to std::vector<HloInstruction*>. This CL replaces this with a proper class which results in better encapsulation of code which deals with schedules and better enforcement of invariants.

This CL also fixes a corner-case bug in dataflow analysis, where values of instructions which are live out of the computation erroneously did not interfere with the values of instructions scheduled after the root instruction.

PiperOrigin-RevId: 211656888