cleanup: scc_iterator consumers should use isAtEnd

No functional change.  Updated loops from:

    for (I = scc_begin(), E = scc_end(); I != E; ++I)

to:

    for (I = scc_begin(); !I.isAtEnd(); ++I)

for teh win.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@200789 91177308-0d34-0410-b5e6-96231b3b80d8