[UnrollAnalyzerTest] Remove dependency to pass managers (#78473)

Remove use of LegacyPassManager in the UnrollAnalyzerTest unit test.

Given that the goal isn't to test pass manager interfaces, and since the
LoopUnrollAnalyzer isn't even implemented as a pass, we do not really
need the complexity of using a pass manager. Instead we just make sure
that we run LoopUnrollAnalyzer and other needed analyses standalone
(without any pass manager). This was inspired by the LoopInfoTest unit
test.