* Fix bug: test/Regression/Transforms/ADCE/2002-07-17-AssertionFailure.ll

  which was caused by us not running the "drop references of dead
  instructions" code when there were no basic blocks that were dead.

  We still don't want to do some of the stuff we do if there are dead
  basic blocks, but we DO want to drop references of dead instructions,
  so we factor the common code out to a new method.

llvm-svn: 3137