[PlaceSafepoints] Switch to being a FunctionPass

The pass doesn't actually modify the module outside of the function being processed. The only confusing piece is that it both inserts calls and then inlines the resulting calls. Given that, it definitely invalidates module level analysis results, but many FunctionPasses do that.

Differential Revision: http://reviews.llvm.org/D9590

llvm-svn: 237185