[polly] [Polly] Guard ISL ast gen compute out (PR #201859)

[Michael Kruse via llvm-commits]

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Meinersbur wrote:

```suggestion
; RUN: opt %loadNPMPolly %s -passes='polly-custom<ast>' -polly-process-unprofitable \
```
[critical] Otherwise it fails in certain build configurations

https://github.com/llvm/llvm-project/pull/201859