[llvm] [LoopVectorize] Enable vectorisation of early exit loops with live-outs (PR #120567)

[David Sherwood via llvm-commits]

david-arm wrote:

PR #88385 is very out of date now, but it also dealt with loops that require a scalar epilogue. I've not attempted to deal with those in this PR, but I am finishing off a downstream patch to handle these too, which will mean we can start vectorising loops in tests/Transforms/LoopVectorizer/multi_early_exit* as well.

I also intend to put up another patch soon that adds support for versioning early exit loops that contain a single load instruction, requiring the correct alignment to ensure safety.

https://github.com/llvm/llvm-project/pull/120567