[libFuzzer] change the way trace-based mutations are applied. Instead of a custom...

[libFuzzer] change the way trace-based mutations are applied. Instead of a custom code just rely on the automatically created dictionary

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@257248 91177308-0d34-0410-b5e6-96231b3b80d8