Added test cases for the newly added LLVM operations and lowering features.

Closes #300

COPYBARA_INTEGRATE_REVIEW=https://github.com/tensorflow/mlir/pull/300 from dfki-jugr:std_to_llvm da6168bbc1a369ae2e99ad3881fdddd82f075dd4
PiperOrigin-RevId: 286231169
Change-Id: I364bda2adad4054052a3a53b4cb1c3de1102864f