[PlaceSafepoints] Make internal helper pass a FunctionPass

Switch from using a LoopPass to using a FunctionPass for the internal helper analysis pass. The next step is going to be to make this a true analysis pass which is required by the PlaceSafepoints pass itself.

p.s. The interesting semantic part here is that we're changing the iteration order over the loops. It shouldn't matter, but that's the reason to separate this into it's own distinct patch.

Differential Revision: http://reviews.llvm.org/D9588

llvm-svn: 237180