JIT: Not all return buffers are marked with DNER when we get to regular promotion

[jakobbotsch]

When we do not mark these with DNER before promotion it results in dependent promotion, instead of allowing physical promotion to handle them. One example of this seems to be Microsoft.CodeAnalysis.CSharp.Symbols.ParameterSymbol:Microsoft.Cci.IParameterTypeInformation.get_CustomModifiers in benchmarks.run_pgo. It seems to be a path that is related to GDV that ends up creating a call node with a return buffer that isn't marked DNER.

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