ppc64le-mlir-rhel-clang intermittently failing in check-mlir

See, for example, https://lab.llvm.org/buildbot/#/builders/129/builds/38552 :

```
command timed out: 1200 seconds without output running [b'ninja', b'check-mlir'], attempting to kill
process killed by signal 9
program finished with exit code -1
```

Seems like it's failing more frequently recently.

CC @llvm/issue-subscribers-backend-powerpc (not sure who to CC for the buildbot).