[SCEV] Use a "Discovered" set instead of a "Visited" set; NFC
[SCEV] Use a "Discovered" set instead of a "Visited" set; NFC

Suggested by Max Kazantsev in https://reviews.llvm.org/D39361

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@319679 91177308-0d34-0410-b5e6-96231b3b80d8

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