[llvm] [DAG] isKnownNeverZero - add ISD::SHL DemandedElts handling and tests (PR #183772)

[Simon Pilgrim via llvm-commits]

RKSimon wrote:

> llvm/utils/update_llc_test_checks.py

Yes - make sure the correct llc is in the path!

```
update_llc_test_checks.py --llc-binary=<path_to_latest_llc> known-never-zero.ll
```

https://github.com/llvm/llvm-project/pull/183772