[PM] Rework a loop in the CGSCC update logic to be more conservative and clear. The current RefSCC can occur in exactly one position so we should just enforce that and leverage the property rather than checking for it anywhere.
[PM] Rework a loop in the CGSCC update logic to be more conservative and
clear. The current RefSCC can occur in exactly one position so we should
just enforce that and leverage the property rather than checking for it
anywhere.

This addresses review comments made on another patch.

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@290162 91177308-0d34-0410-b5e6-96231b3b80d8

apple/swift-llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,llvm-mirror/llvm,llvm-mirror/llvm,llvm-mirror/llvm,llvm-mirror/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm