[test] Rewrite phi-empty.ll into a unittest

phi-empty.ll does not pass under the new PM because the NPM runs
-loop-simplify. Running -loop-simplify ends up not reproing
https://llvm.org/PR48296.

Verified that this test fails when 9eb2c011 is reverted.

Reviewed By: spatel

Differential Revision: https://reviews.llvm.org/D92807