[release/9.0-staging] Do not substitute return values of constrained calls

[github-actions]

Backport of #111030 to release/9.0-staging

/cc @MichalStrehovsky

Customer Impact

• Customer reported
• Found internally

This was hit by two customers: #110932 and #113423.

Certain static interface method invocation patterns can get miscompiled into an infinite loop. Instead of calling the method, we loop forever.

Regression

• Yes
• No

Regressed when we added more dead code elimination. Dead code elimination incorrectly proves the branch as unreachable.

Testing

This was missed because it requires some luck to hit:

• Static interface method call needs to be part of an if check.
• The static interface method needs to have a default implementation.
• The default implementation needs to be simple enough that we can statically analyze it.

We now have a regression test for this pattern.

Risk

Low: this has been fixed in the .NET 10 branch for two months and the fix is obvious (in retrospect).

IMPORTANT: If this backport is for a servicing release, please verify that:

• The PR target branch is release/X.0-staging, not release/X.0.

Package authoring no longer needed in .NET 9

IMPORTANT: Starting with .NET 9, you no longer need to edit a NuGet package's csproj to enable building and bump the version.
Keep in mind that we still need package authoring in .NET 8 and older versions.

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[jeffschwMSFT]

lgtm. we will take for consideration in 9.0.x