[llvm] LoopVectorize: add test for crash in #72969 (PR #74111)

[Ramkumar Ramachandra via llvm-commits]

artagnon wrote:

I got a [buildbot failure](https://lab.llvm.org/buildbot/#/builders/98/builds/33789) post-merge, although the native CI seems to have passed it.

https://github.com/llvm/llvm-project/pull/74111