[llvm] [SystemZ] Modify tests for constrained rounding functions (PR #116952)

[Ulrich Weigand via llvm-commits]

https://github.com/uweigand requested changes to this pull request.

This removes the recently added atan2 tests, which looks like an oversight?

Also, according to the CI runs the test now fails with:
```
<stdin>:4433:43: error: '%b' defined with type '<3 x double>' but expected '<3 x float>'
                              <3 x float> %b,
```

https://github.com/llvm/llvm-project/pull/116952