srtree-3.0.0.2 bench needs llvm

```text
While building package srtree-3.0.0.2 using:
/home/curators/.stack/setup-exe-cache/x86_64-linux-tinfo6/Cabal-simple_w2MFVN35_3.14.2.0_ghc-9.12.4 --verbose=1 --builddir=.stack-work/dist/x86_64-linux-tinfo6/ghc-9.12.4 build lib:srtree exe:bench exe:bench-eqsat exe:srtree-report test:srtree-test --ghc-options " -fdiagnostics-color=always"
Process exited with code: ExitFailure 1

Preprocessing executable 'bench' for srtree-3.0.0.2...
Building executable 'bench' for srtree-3.0.0.2...
: error:
Warning: Couldn't figure out LLVM version!
Make sure you have installed LLVM between [13 and 20)

: error:
LLVM Optimiser: could not execute: opt-20
```

Start with the srtree build command and the executable entry points named in the report: bench, bench-eqsat, srtree-report, and test:srtree-test. Reproduce the failure and inspect how the bench build invokes LLVM, especially the missing opt-20 executable; done means the reported package build completes successfully.

Written by the indexing model from the issue text.