[Port] Fix the incorrect merge logic for back edges for GlobalFlowStateAnalysis

[mavasani]

Ports #6029 to .NET6 release branch.

Customer Impact

A customer reported the issue in VS feedbacks which transferred to github: #6015 and it is reproduceable with the linked project.

When builds the project with CA1416 Platform Compatibility Analyzer enabled then the build never ends and memory grows continually, even stopping the build and closed the VS the VBCSCompiler process stays alive and memory keep growing, only option is kill the process

CA1416 is included in SDK and enabled by default, therefore it could happen for any customer's project that happens to have a code section that reproes the issue.

Testing

• Unit tested verified that prior to this fix, the added unit test hang due to this issue.
• Manually tested with the repro: verified that building the github repo cited in the feedback ticket with platform compat analyzer also leads to a hang before this fix. Both these repros are fixed after this PR.

Risk

Very low - GlobalFlowStateAnalysisValueSet.Unset is used to represent unanalyzed flow state, GlobalFlowStateAnalysisValueSet.Empty is used to represent analyzed flow state with no analysis values. We initialize the flow states for the entry basic block of the CFG with Unset value, but never replace the Unset with Empty for subsequent passes. This in turn leads to incorrect merge when a Known value flows from the back edge to the loop start and ends up overriding the Unset value instead of being overridden by the Empty value. Now we correctly perform this merge logic.

[mavasani]

@JoeRobich @jmarolf - seems like we need to port #6032 to release/6.0.4xx branch to fix the build errors about duplicate package references.

[jeffhandley]

Can those changes be merged into this branch/PR since otherwise they wouldn't need to be ported?

[mavasani]

Thanks @JoeRobich!

[buyaa-n]

Thanks @mavasani I have updated the description by the 6.0 servicing template, please check and updates as needed, especially the Risk part might need more update

[mavasani]

Thanks @buyaa-n - I believe your description is appropriate, we don't need further updates.

[mavasani]

@JoeRobich @jmarolf Do you know what is causing the build failures? Are we using the correct .NET SDK/compiler for the build?

.dotnet\sdk\6.0.100-rc.1.21430.12\Sdks\Microsoft.NET.Sdk\targets\Microsoft.NET.Sdk.targets#L1088
.dotnet\sdk\6.0.100-rc.1.21430.12\Sdks\Microsoft.NET.Sdk\targets\Microsoft.NET.Sdk.targets(1088,5): error MSB4018: (NETCORE_ENGINEERING_TELEMETRY=Build) The "ValidateExecutableReferences" task failed unexpectedly.
System.NullReferenceException: Object reference not set to an instance of an object.
   at Microsoft.NET.Build.Tasks.ValidateExecutableReferences.ExecuteCore()
   at Microsoft.NET.Build.Tasks.TaskBase.Execute()
   at Microsoft.Build.BackEnd.TaskExecutionHost.Microsoft.Build.BackEnd.ITaskExecutionHost.Execute()
   at Microsoft.Build.BackEnd.TaskBuilder.<ExecuteInstantiatedTask>d__26.MoveNext()

[codecov]

Codecov Report

Merging #6038 (a7abd5e) into release/6.0.4xx (2b525af) will increase coverage by 0.00%.
The diff coverage is 92.10%.

@@               Coverage Diff                @@
##           release/6.0.4xx    #6038   +/-   ##
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  Coverage            95.53%   95.53%           
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  Files                 1275     1275           
  Lines               292697   292730   +33     
  Branches             17686    17688    +2     
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+ Hits                279634   279671   +37     
- Misses               10655    10657    +2     
+ Partials              2408     2402    -6     

[buyaa-n]

Thanks @mavasani @JoeRobich the 6.0 servicing request approved over e-mail. Feel free to merge!