[test] Cleanup some CGSCCPassManager tests
Don't iterate over SCC as we potentially modify it.
Verify module (and fix some broken ones).
Only run pass once and make sure that it's actually run.
Rename tests to just end in a number since I'm planning on adding a
bunch more which won't have good individual names. Instead, add comments
on the transformations that each test does.
Reviewed By: rnk
Differential Revision: https://reviews.llvm.org/D93427