Fix BigInteger.Rotate{Left,Right} for backport

[kzrnm]

Fix #112854

In #112864, the whole algorithm is rewritten for .NET 10, but the changes in this PR are minimal.

[tannergooding]

/backport to release/9.0

[github-actions]

Started backporting to release/9.0: https://github.com/dotnet/runtime/actions/runs/13572166557

[tannergooding]

/backport to release/8.0

[github-actions]

Started backporting to release/8.0: https://github.com/dotnet/runtime/actions/runs/13572171653

[jeffhandley]

@kzrnm Can you clarify your backport requirements for this fix please? Would backporting only to .NET 9 satisfy your needs, or are you still targeting .NET 8?

[kzrnm]

@jeffhandley cc. @tannergooding
.NET 8 would also need to be backported. This bug has existed since the method was introduced in .NET 7 and should be fixed in all supported versions.

[jeffhandley]

Thanks, @kzrnm. We're discussing the backports and trying to balance risk. It would help us to better understand if your scenario is blocked by .NET 8 having this issue, or if backporting only to .NET 9 would unblock you. If you are targeting .NET 9, then we might backport to .NET 9 first and give the fix some more bake time there before backporting to .NET 8.

Are you targeting .NET 8 or .NET 9 in your scenario? Thanks again.

[kzrnm]

I created the PR for backporting based on #112864 (comment), so I want to leave the decision on whether it should be backported to @tannergooding.

To be honest, I don’t have a scenario where I use this method. I reported the bug simply because it returns an obviously incorrect value.