[llvm] [LegalizeDAG] Optimize CodeGen for `ISD::CTLZ_ZERO_UNDEF` (PR #83039)

[Jay Foad via llvm-commits]

jayfoad wrote:

> I do not understand why the CI failed here: https://buildkite.com/llvm-project/github-pull-requests/builds/76944#019068e3-bd93-4ad9-ba55-817fdabbe0eb. Looks like all the tests passed

Further up in the build log there is:
```
Failed Tests (1):
  BOLT :: X86/reader-stale-yaml-std.test
```
and:
```
Timed Out Tests (1):
  lldb-api :: functionalities/fork/concurrent_vfork/TestConcurrentVFork.py
```

https://github.com/llvm/llvm-project/pull/83039