[NewPM] Disable PreservedCFGChecker and add regression unit tests

The design of the PreservedCFG Checker (landed with the commit
28012e00) has a fundamental flaw which makes it incorrect.
The checker is based on the PreservedAnalyses result returned
by functional passes: if CFGAnalyses is in the returned
PreservedAnalyses set, then the checker asserts that the CFG
snapshot saved before the pass is equal to the CFG snapshot
taken after the the pass. The problem is in passes that change
CFG and invalidate CFGAnalyses on their own. Such passes do not
return CFGanalyses in the returned PreservedAnalyses. So the
checker mistakenly expects CFG unchanged. As an example see the
class TestSimplifyCFGInvalidatingAnalysisPass in the new tests.

It is interesting that the bug was not found in LLVM. That is
because the CFG checker ran only if CFGAnalyses was checked
incorrectly:
  if (!PassPA.allAnalysesInSetPreserved<CFGAnalyses>())
    return;

but must be checked as follows:
  auto PAC = PA.getChecker<PreservedCFGCheckerAnalysis>();
  if (!(PAC.preserved() ||
        PAC.preservedSet<AllAnalysesOn<Function>>() ||
        PAC.preservedSet<CFGAnalyses>())
    return;

A fully redesigned checker will be sent as a separate follow-up
patch.

Reviewed By: Serguei Katkov, Jakub Kuderski

Differential Revision: https://reviews.llvm.org/D91324