Factor out MachineBlockPlacement::fillWorkLists. NFC

Summary: There are places in MachineBlockPlacement where a worklist is filled in pretty much identical way. The code is duplicated. This refactor it so that the same code is used in both scenarii.

Reviewers: chandlerc, majnemer, rafael, MatzeB, escha, silvas

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D18077

llvm-svn: 263495