[LibFuzzer] Tweak `MutationDispatcher::Mutate_CopyPart` mutation.

It doesn't make sense to non-deterministically choose between
`CopyPart(..)` and `InsertPart(..)` when it is known that
`InsertPart(..)` will fail.

This upstream's a change from JFS solver's fork of LibFuzzer.

Differential Revision: https://reviews.llvm.org/D45693

llvm-svn: 330687