Copy live sets when splitting blocks (dotnet/coreclr#26809)

Copy live sets when splitting blocks (dotnet/coreclr#26809)

With dotnet/coreclr#26456 we may generate labels for split blocks (even though there's no associated code). Therefore we need to ensure that the live sets are correct.
Also, cleanup some miscellaneous dumping code.

Fix dotnet/coreclr#26795

Commit migrated from https://github.com/dotnet/coreclr/commit/7ff9851cdee43f9ae5bec5f1c28f52609526542e