[llvm] [MemCpyOpt] Fix infinite loop in memset+memcpy fold (PR #98638)

[Nikita Popov via llvm-commits]

nikic wrote:

For the record, https://github.com/llvm/llvm-project/commit/07460b66666403fc86e01d3fed6b77c1baf94cb1 was my previous attempt to fix this.

https://github.com/llvm/llvm-project/pull/98638