[opt-viewer] Avoid division by zero
[opt-viewer] Avoid division by zero

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@286172 91177308-0d34-0410-b5e6-96231b3b80d8
(cherry picked from commit 3fbb1a90c3b46d3796c58001ff05781d8506a2ad)

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