Do not keep cctor for instance fields (dotnet#96656)

src/tools/illink/src/linker/Linker.Steps/MarkStep.cs

@@ -1805,7 +1805,7 @@ void MarkField (FieldDefinition field, in DependencyInfo reason, in MessageOrigi
 			}
 
 			var parent = field.DeclaringType;
-			if (!Annotations.HasPreservedStaticCtor (parent)) {
+			if (field.IsStatic && !Annotations.HasPreservedStaticCtor (parent)) {
 				var cctorReason = reason.Kind switch {
 					// Report an edge directly from the method accessing the field to the static ctor it triggers
 					DependencyKind.FieldAccess => new DependencyInfo (DependencyKind.TriggersCctorThroughFieldAccess, reason.Source),

src/tools/illink/test/Mono.Linker.Tests.Cases/Statics/UnusedStaticFieldInitializer.cs

@@ -7,6 +7,8 @@ public class UnusedStaticFieldInitializer
 		public static void Main ()
 		{
 			C.Foo ();
+
+			new C2 (123);
 		}
 
 		static class C
@@ -18,5 +20,16 @@ public static void Foo ()
 			{
 			}
 		}
+
+		class C2
+		{
+			public static object o = new object ();
+
+			[Kept]
+			public int Field;
+
+			[Kept]
+			public C2 (int val) => Field = val;
+		}
 	}
 }