[SimplifyCFG] add a struct to house optional folds (PR34603)

[SimplifyCFG] add a struct to house optional folds (PR34603)

This was intended to be no-functional-change, but it's not - there's a test diff.

So I thought I should stop here and post it as-is to see if this looks like what was expected
based on the discussion in PR34603:
https://bugs.llvm.org/show_bug.cgi?id=34603

Notes:
 1. The test improvement occurs because the existing 'LateSimplifyCFG' marker is not carried
    through the recursive calls to 'SimplifyCFG()->SimplifyCFGOpt().run()->SimplifyCFG()'.
    The parameter isn't passed down, so we pick up the default value from the function signature
    after the first level. I assumed that was a bug, so I've passed 'Options' down in all of the
    'SimplifyCFG' calls.

 2. I split 'LateSimplifyCFG' into 2 bits: ConvertSwitchToLookupTable and KeepCanonicalLoops.
    This would theoretically allow us to differentiate the transforms controlled by those params
    independently.

 3. We could stash the optional AssumptionCache pointer and 'LoopHeaders' pointer in the struct too.
    I just stopped here to minimize the diffs.

 4. Similarly, I stopped short of messing with the pass manager layer. I have another question that
    could wait for the follow-up: why is the new pass manager creating the pass with LateSimplifyCFG
    set to true no matter where in the pipeline it's creating SimplifyCFG passes?

    // Create an early function pass manager to cleanup the output of the
    // frontend.
    EarlyFPM.addPass(SimplifyCFGPass());

    -->

    /// \brief Construct a pass with the default thresholds
    /// and switch optimizations.
    SimplifyCFGPass::SimplifyCFGPass()
       : BonusInstThreshold(UserBonusInstThreshold),
         LateSimplifyCFG(true) {}   <-- switches get converted to lookup tables and loops may not be in canonical form

    If this is unintended, then it's possible that the current behavior of dropping the 'LateSimplifyCFG'
    setting via recursion was masking this bug.

Differential Revision: https://reviews.llvm.org/D38138

llvm-svn: 314308