Revert "Handle casts done via helpers and fold overflow operations value numbering (#50450)"

[SingleAccretion]

This is causing too many failures for me to investigate in time (#51381, #51380), so reverting the change wholesale.

I am planning to revisit this with the approach of not folding UB casts.

Note that UB means Undefined Behavior

cc @BruceForstall, @tannergooding

[tannergooding]

This LGTM. However, is there a smaller change we can make to just block the new functionality enabled by the PR rather than reverting the entire thing?

If not, that's fine.

[SingleAccretion]

I think so, yes, there is a small change that I could make here (block folding for all UB casts, not just checked ones, in gtFoldExprConst, and the same in VN), but it does require me to make sure all the failing tests pass, and I am afraid I will not be able to make sure that is the case in time.

Note on the formatting failure:

ERROR: failed to download clang-tidy.

this is the second time I am seeing this...

[briansull]

I approve this revert

[briansull]

The jit-format on windows x64 failed due to an infrastructure glitch:

2021-04-16T16:55:44.9456599Z Invoke-WebRequest : Unable to read data from the transport connection: An existing connection was forcibly closed by 
2021-04-16T16:55:44.9463469Z the remote host.
2021-04-16T16:55:44.9464753Z At line:1 char:1
2021-04-16T16:55:44.9466317Z + Invoke-WebRequest -Uri https://clrjit.blob.core.windows.net/clang-too ...
2021-04-16T16:55:44.9468097Z + ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
2021-04-16T16:55:44.9470134Z     + CategoryInfo          : NotSpecified: (:) [Invoke-WebRequest], IOException
2021-04-16T16:55:44.9471879Z     + FullyQualifiedErrorId : System.IO.IOException,Microsoft.PowerShell.Commands.InvokeWebRequestCommand
2021-04-16T16:55:44.9473964Z  
2021-04-16T16:55:44.9810889Z ERROR: failed to download clang-tidy.

[briansull]

I asked that the Formatting x64 task be re-run, and it acknowledges it but I didn't see it start the re-run.

[briansull]

The Linux release failures issue this message:

2021-04-16T18:34:59.7566163Z XUnit : error : Tests failed: /root/runtime/artifacts/TestResults/Release/Microsoft.NET.HostModel.AppHost.Tests_net6.0_x64.html [net6.0|x64] [/root/runtime/src/installer/tests/Microsoft.NET.HostModel.Tests/Microsoft.NET.HostModel.AppHost.Tests/Microsoft.NET.HostModel.AppHost.Tests.csproj]

This is a known failure:" #51372