Pass to reduce added flags from RemoveReturn

[ChrisDodd]

This adds a pass running before RemoveReturns that rewrites code with a conditional jump (a break/continue/exit/return in a if branch) where there's code after the if statement that will only run if the if condition is false. It moves that following code into the else (ifFalse) part of the IfStatement

The point of this is avoid needing to introduce a flag variable in the later RemoveReturns or RemoveExits. It can also help with loops.

[jafingerhut]

@fruffy Does something like gauntlet run automatically on all PRs? Or is there a way to manually trigger it on a particular PR by adding a tag? This PR seems likes a good one to have that extra checking, if it is easy to enable.

[fruffy]

@fruffy Does something like gauntlet run automatically on all PRs? Or is there a way to manually trigger it on a particular PR by adding a tag? This PR seems likes a good one to have that extra checking, if it is easy to enable.

Yes, I added the run-validation tag which will run the tool once the pull request is updated. It will also run nightly after it has been merged.

[jafingerhut]

Caveats: I did not review the C++ code changes. I also did not carefully review most of the expected output P4C changes. They are simply too long to do so in a reasonable amount of time, hence my question of Fabian about adding the run-validation tag to get better automated coverage of the correctness of those changes.

Thus this is a very "weak" LGTM approval, based upon me manually checking a few of the very short expected output files, and finding no problems.

If someone is willing to review parts of this PR that I have not, I would consider my approval to be not enough on its own to merge this.

[ChrisDodd]

Turns out there's a bug here (incorrectly applying changes) due to ignoring the effects of switch statements (and loops), so I'll push a revision soon.

[vlstill]

I like this. I've noticed the RemoveRetun pass introduces way too much flags, but did not get to figuring out why and what we can do about this :-). I just have some questions.

[vlstill]

Maybe just also rename hasBranched to hasJumped or something like that.