[ThinLTO] Fix bot failure due to unused variable with NDEBUG
[ThinLTO] Fix bot failure due to unused variable with NDEBUG

Put variable only used in assert under #ifndef NDEBUG.

This should fix bot failure at
http://lab.llvm.org:8011/builders/lld-x86_64-darwin13/builds/28537

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@284039 91177308-0d34-0410-b5e6-96231b3b80d8

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