Refine the DefUseManager

Refine the DefUseManager

* Fix the behavior when analyzing an individual instruction:
  * exisiting instruction:
      Clear the original records and re-analyze it as a new instruction.
  * new instruction with exisiting result id:
      Clear the original records of the exisiting result id. This means
      the records of the analyzed result-id-defining instruction will be
      overwritten by the record of the new instruction with the same
      result id.
  * new instruction with new result id or without result id:
      Just update the internal records to incorperate the new
      instruction.
* Add tests for analyzing individual instruction w/o an exisiting module.
* Refactor ClearInst() implementation
* Remove ClearDef() function.
* Fixed a bug in DefUseManager::ReplaceAllUsesWith() that OpName
  instruction may trigger the assertion incorrectly.
* update the blurbs for EraseUseRecordsOfOperandIds()