Enhance the InstrStage object to enable the specification of an Itinerary with overlapping stages. The default is to maintain the current behavior that the "next" stage immediately follows the previous one.

llvm-svn: 78827

llvm-svn: 78736

 - Since the function is never called in NDEBUG mode, just dropped the DEBUG()
   uses here.

llvm-svn: 78649

llvm-svn: 78634

Post RA scheduler changes. Introduce a hazard recognizer that uses the target schedule information to accurately model the pipeline. Update the scheduler to correctly handle multi-issue targets.

llvm-svn: 78563