building on the new CallGraphSCC abstraction, teach CallGraphSCCPassManager

building on the new CallGraphSCC abstraction, teach CallGraphSCCPassManager
to keep the node entries in scc_iterator up to date instead of dangling as
the SCC mutates.

This is a really terrible problem which was causing -g to affect codegen
because it would permute the memory image of the compiler process.

Thanks to Dale for expertly hunting it down.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@101565 91177308-0d34-0410-b5e6-96231b3b80d8