Update IntrospectionExtensions.cs (dotnet/coreclr#8029)

Update IntrospectionExtensions.cs (dotnet/coreclr#8029)

1. A few tweaks were made to follow the [Coding Style](https://github.com/dotnet/corefx/blob/master/Documentation/coding-guidelines/coding-style.md).
2. A `null` check was done after doing a simple cast (`(IReflectableType)type`), but `rcType` cannot be `null` (it could only be `null` if a [`as`](https://msdn.microsoft.com/en-us/library/cc488006.aspx) cast was done).

Commit migrated from https://github.com/dotnet/coreclr/commit/55a89556b53feeb6fcaa15ae48305994e690f2e0