[clang] [llvm] [SROA] Vector promote some memsets (PR #133301)

[Matt Arsenault via llvm-commits]

arsenm wrote:

> @arsenm Any recommendations for appeasing the [undef deprecator](https://github.com/llvm/llvm-project/pull/133301#issuecomment-2759145421)?

I don't think you did anything other than update existing tests, I would ignore it for the purposes of this change 

https://github.com/llvm/llvm-project/pull/133301