C#: Restrict post-dominance to normal execution

[hvitved]

Overview

As discussed several times with @aschackmull, post-dominance makes more sense when restricted to paths that leave the callable normally, that is, without throwing an exception or halting.

For example, in

void M3(string s)
{
    if (s is null)
        throw new ArgumentNullException(nameof(s));
    Console.WriteLine(s);
}

the call Console.WriteLine(s) will happen on all normal paths through M3, so we want it to post-dominate the entry point.

Implementation

Before this PR, the CFG for M3 was

and post-dominance was defined in terms of the exit M3 node.

This PR changes the CFG for M3 to

and redefines post-dominance with respect to the new exit M3 (normal) node that represents normal execution (highlighted in red).

Note that each callable will still have a unique exit node, which is reached from the two new normal/abnormal exit nodes: this is for backwards compatibility, and so that we can recover the old post-dominance logic, if needed.

CSharp-Differences here.

[tamasvajk]

These CFG diagrams are super useful.

The differences job is red.

[hvitved]

The differences job is red.

Yeah, I am not quite sure why that is, as it has run all projects successfully and done the comparison. So I think we can just ignore it.