[llvm] [AArch64] Cap upper-bound unrolling of loops with uncomputable trip counts (PR #205102)

[David Sherwood via llvm-commits]

david-arm wrote:

Right I understand this a bit more now. Thanks for showing the full diff here @igogo-x86 - it really helps highlight the problem! This PR highlights an underlying bug with the vectoriser where it's not safe to pass around ScalarEvolution pointers after forgetting the loop. If we pass the `SE` pointer to `getUnrollingPreferences` it's not entirely unreasonable that the caller might actually want to use it. :)

https://github.com/llvm/llvm-project/pull/208500 fixes the underlying bug exposed by this PR and once it lands the extra loop vectoriser test diffs in this PR will disappear.

https://github.com/llvm/llvm-project/pull/205102