Reduced memory alloc for combined binding subjects

[YohDeadfall]

What does the pull request do?

Removes one allocation and additional virtual calls caused by needles creation of CombinedSubject<T> when both the observer and the observable are the same object.

What is the current behavior?

Always create a wrapper.

What is the updated/expected behavior with this PR?

No behavior change at all.

Checklist

• Added unit tests (if possible)?
• Added XML documentation to any related classes?
• Consider submitting a PR to https://github.com/AvaloniaUI/avalonia-docs with user documentation

[avaloniaui-bot]

You can test this PR using the following package version. 11.2.999-cibuild0051451-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

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[maxkatz6]

LGTM