Revert "CallGraphSCCPass: iterate over all functions."

This reverts commit r336419: use-after-free on CallGraph::FunctionMap elements
due to the use of a stale iterator in CGPassManager::runOnModule.

The iterator may be invalidated if a pass removes a function, ex.:
  llvm::LegacyInlinerBase::inlineCalls
  inlineCallsImpl
  llvm::CallGraph::removeFunctionFromModule

llvm-svn: 337018