[libFuzzer] add InsertRepeatedBytes and EraseBytes.

New mutation: InsertRepeatedBytes.
Updated mutation: EraseByte => EraseBytes.

This helps https://github.com/google/sanitizers/issues/710
where libFuzzer was not able to find a known bug.
Now it finds it in minutes.

Hopefully, the change is general enough to help other targets.

llvm-svn: 278687