[Xamarin.Android.Build.Tasks] Small refactoring to AddKeepAlives method (#8423)

The `method.Parameters.Count == 0` check should be faster than LINQ's `Any` call
(`CustomAttributes.Any`). So moved it first.

Moved some variables near their usage, particularly, `GetILProcessor` which may
allocate unnecessarily if `GC.KeepAlive(...)` call was already found. It's not a
big of problem though as it's not very common.

Author: Youssef1313

src/Xamarin.Android.Build.Tasks/Linker/MonoDroid.Tuner/AddKeepAlivesStep.cs

@@ -86,18 +86,13 @@ bool AddKeepAlives (TypeDefinition type)
 		{
 			bool changed = false;
 			foreach (MethodDefinition method in type.Methods) {
-				if (!method.CustomAttributes.Any (a => a.AttributeType.FullName == "Android.Runtime.RegisterAttribute"))
+				if (method.Parameters.Count == 0)
 					continue;
 
-				if (method.Parameters.Count == 0)
+				if (!method.CustomAttributes.Any (a => a.AttributeType.FullName == "Android.Runtime.RegisterAttribute"))
 					continue;
 
-				var processor = method.Body.GetILProcessor ();
-				var module = method.DeclaringType.Module;
 				var instructions = method.Body.Instructions;
-				var end = instructions.Last ();
-				if (end.Previous.OpCode == OpCodes.Endfinally)
-					end = end.Previous;
 
 				var found = false;
 				for (int off = Math.Max (0, instructions.Count - 6); off < instructions.Count; off++) {
@@ -111,6 +106,12 @@ bool AddKeepAlives (TypeDefinition type)
 				if (found)
 					continue;
 
+				var processor = method.Body.GetILProcessor ();
+				var module = method.DeclaringType.Module;
+				var end = instructions.Last ();
+				if (end.Previous.OpCode == OpCodes.Endfinally)
+					end = end.Previous;
+
 				for (int i = 0; i < method.Parameters.Count; i++) {
 					if (method.Parameters [i].ParameterType.IsValueType || method.Parameters [i].ParameterType.FullName == "System.String")
 						continue;