[llvm] [FuzzMutate] Prevent the mutator from generating illegal memory operations (PR #144885)

[Manuel Carrasco via llvm-commits]

mgcarrasco wrote:

@DataCorrupted thanks again for your review and feedback. I've addressed the remaining comments. In case the latest changes look good to you, I think the PR could be merged. 

https://github.com/llvm/llvm-project/pull/144885