[llvm] [LV] Initial support for stores in early exit loops (PR #137774)

[Graham Hunter via llvm-commits]

huntergr-arm wrote:

I've started using vplan pattern matchers in the transform; still feels like there's potential for it to exclude a bunch of valid loops if something is slightly different.

At this point I think I'll precommit the tests and separate out some parts into additional PRs.

https://github.com/llvm/llvm-project/pull/137774