[HotColdSplit] Remove a set which tracked split functions (NFC)
[HotColdSplit] Remove a set which tracked split functions (NFC)

Use the begin/end iterator idiom to avoid visiting split functions,
instead of doing a set lookup.

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@351622 91177308-0d34-0410-b5e6-96231b3b80d8
(cherry picked from commit 1054bdf35560e528d67369213bec852c0c29373d)

apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm