[llvm] [LV] Add early-exit-with-store tests (PR #140899)

[David Sherwood via llvm-commits]

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david-arm wrote:

It's probably worth adding these tests too:

1. A test for a store to an invariant address if that matters for your early exit work.
2. A loop with a store in the latch block instead of the early exiting block.
3. A test with a store to an unknown address that isn't safe to vectorise.
4. A test with a store where the loop condition is invariant (since I believe this will be important for your follow-on work?).
5. Do we need a test that requires runtime memory checks? I don't know if this is possible, but if two `ptr dereferenceable(40)` pointer arguments are passed they could still potentially alias, even if it's something as simple as they are just identical pointers. I just want to make sure we don't silently vectorise such a loop without runtime checks.

https://github.com/llvm/llvm-project/pull/140899