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

[Nikita Popov via llvm-commits]

nikic wrote:

And this is why I told you not to pre-commit tests for crashes :) If you do this, you need to at least make it a `REQUIRES: asserts` test, to be reasonably confident that it will always crash.

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