verify-uselistorder: Change the default -num-shuffles=5

Change the default for `-num-shuffles` to 5 and better document the
algorithm in the header docs of `verify-uselistorder`.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@214419 91177308-0d34-0410-b5e6-96231b3b80d8