[llvm] [MemCpyOpt] Handle scalable aggregate types in memmove/memset formation (PR #80487)

[Alex Bradbury via llvm-commits]

asb wrote:

@preames I believe this is a candidate for backporting? (the test case does indeed seem to crash on the release/18.x branch right now).

https://github.com/llvm/llvm-project/pull/80487