[PM] Basic cleanups to CGSCC update code, NFC.

Just using InstIterator, simpler loop structures, and making better use
of the visit callback infrastructure.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@288790 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed