Do not select EhPad BB in MachineBlockPlacement when there is regular BB to schedule

Summary:
EHPad BB are not entered the classic way and therefor do not need to be placed after their predecessors. This patch make sure EHPad BB are not chosen amongst successors to form chains, and are selected as last resort when selecting the best candidate.

EHPad are scheduled in reverse probability order in order to have them flow into each others naturally.

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

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D17625

llvm-svn: 265726