[CallGraphUpdater] Remove nodes from their SCC (old PM)

Summary:
We can and should remove deleted nodes from their respective SCCs. We
did not do this before and this was a potential problem even though I
couldn't locally trigger an issue. Since the `DeleteNode` would assert
if the node was not in the SCC, we know we only remove nodes from their
SCC and only once (when run on all the Attributor tests).

Reviewers: lebedev.ri, hfinkel, fhahn, probinson, wristow, loladiro, sstefan1, uenoku

Subscribers: hiraditya, bollu, uenoku, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D77855