Fix ParallelTaskAssigner's use of the HloPassPipeline interface

We were creating the ParallelTaskAssignment contained in ParallelTaskAssigner
with an unoptimized module and then trying to ParallelTaskAssigning::Run on an
optimized module.  This meant that the flop counts in HloCostAnalysis were
cached using bogus HloInstruction* pointers, which meant our parallel task
assignment was not effective.

PiperOrigin-RevId: 174909618