C#: Implement CFG for not patterns

[hvitved]

This PR changes the CFG for pattern expressions (including not expressions). Example:

public static bool M3(object c) =>
    c is not null ? c is 1 : c is 2;

CFG before:

CFG after:

https://jenkins.internal.semmle.com/job/Changes/job/CSharp-Differences/782/
https://jenkins.internal.semmle.com/job/Changes/job/CSharp-Differences/799/
https://jenkins.internal.semmle.com/job/Changes/job/CSharp-Differences/804/

[tamasvajk]

LGTM

[hvitved]

Rebased and added two new commits.

[hvitved]

Fixed a join-order regression caused by 355edcb, and started a new differences job.

[tamasvajk]

This seems overly complicated. Can you show me what perf issue was solved by this?

[hvitved]

You are right, it is complicated. It solves a bad-join order that was also there before this PR, but which I spotted when fixing the other bad join-order:

                      1986     ~0%       {5} r84 = SCAN ControlFlowGraphImpl::Statements::TryStmtTree::lastFinally_dispred#fffff#prev_delta AS I OUTPUT I.<4>, I.<0> 'this', I.<1> 'last', I.<2> 'c', I.<3>
                      29295324 ~1%       {5} r85 = JOIN r84 WITH Completion::TNestedCompletion#ffff_23#join_rhs AS R ON FIRST 1 OUTPUT R.<1> 'c', r84.<4>, r84.<1> 'this', r84.<2> 'last', r84.<3>
                      10451    ~0%       {4} r86 = JOIN r85 WITH Completion::NestedCompletion::getOuterCompletion_dispred#ff AS R ON FIRST 2 OUTPUT r85.<0> 'c', r85.<4>, r85.<2> 'this', r85.<3> 'last'
                      499      ~3%       {3} r87 = JOIN r86 WITH Completion::NestedCompletion::getAnInnerCompatibleCompletion_dispred#ff AS R ON FIRST 2 OUTPUT r86.<3> 'last', r86.<0> 'c', r86.<2> 'this'

The rewrite avoids joining first on getNestLevel(), so it instead becomes

                      1986    ~0%       {5} r93 = SCAN ControlFlowGraphImpl::Statements::TryStmtTree::lastFinally_dispred#fffff#prev_delta AS I OUTPUT I.<3>, I.<0> 'this', I.<1> 'last', I.<2> 'c', I.<4>
                      28433   ~0%       {5} r94 = JOIN r93 WITH Completion::NestedCompletion::getOuterCompletion_dispred#ff_10#join_rhs AS R ON FIRST 1 OUTPUT R.<1> 'c', r93.<3>, r93.<1> 'this', r93.<2> 'last', r93.<4>
                      1497    ~7%       {4} r95 = JOIN r94 WITH Completion::NestedCompletion::getAnInnerCompatibleCompletion_dispred#ff AS R ON FIRST 2 OUTPUT r94.<0> 'c', r94.<2> 'this', r94.<3> 'last', r94.<4>
                      1497    ~0%       {7} r96 = JOIN r95 WITH Completion::TNestedCompletion#ffff_3012#join_rhs AS R ON FIRST 1 OUTPUT r95.<1> 'this', r95.<2> 'last', r95.<3>, r95.<0> 'c', R.<1>, R.<2>, R.<3>
                      1497    ~0%       {7} r97 = SELECT r96 ON r96.<6> >= r96.<2>
                      499     ~0%       {7} r98 = SELECT r97 ON r97.<6> <= r97.<2>
                      499     ~3%       {3} r99 = SCAN r98 OUTPUT r98.<1> 'last', r98.<3> 'c', r98.<0> 'this'