Add preparePassManager() hook. This allows each pass to check whether

current active pass manager is appropriate or not.

A loop pass may consider current LPPassManager in appropraite if loop
pass is not preserving analysis information that is used by other
passes managed by current LPPassManager. In such situation, loop pass
can pop current LPPassManager from the PMStack using this  hook
and use new LPPassManager for itself.

llvm-svn: 34941