[PhaseOrdering] add tests for bittest patterns from bitfields; NFC

As mentioned in D45986, there's a potential ordering dependency 
between instcombine and aggressive-instcombine for detecting these,
so I'm adding a few tests to confirm that the expected folds occur
using -O3 (because aggressive-instcombine only runs at -O3 currently).

llvm-svn: 331308