Avoid MakeGenericMethod in XmlSerializer

[MichalStrehovsky]

I pulled out the IsValueType part of the generic method into the non-generic method since it doesn't use anything from the generic context. This should be an improvement irrespective of AOT.

Then I made us take the IsValueType path if IsDynamicCodeSupported is false. This has two consequences:

• Avoids MakeGenericType that may or may not succeed with IsDynamicCodeSupported=false
• Avoids LINQ expressions in the field case. This is will be a throughput improvement.
• There might be a small throughput regression for the property case when IsDynamicCodeSupported is false, but we get reliability in return.

Cc @dotnet/ilc-contrib

[eerhardt]

@dotnet/area-system-xml @HongGit @StephenMolloy @mconnew - any feedback here?

[StephenMolloy]

lgtm