[opt-viewer] Avoid duplicated remarks
[opt-viewer] Avoid duplicated remarks

This can happen if a pass is run multiple times or if the code is in a
header file which is included multiple times.

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@286489 91177308-0d34-0410-b5e6-96231b3b80d8
(cherry picked from commit e13ecb7d133f59ef733da5782184b3237b76d3b7)

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