[IR] Move IrValueDeclaration.isAssignable to extension property

to remove custom logic from generated classes, in particular
to simplify source generator itself.

compiler/ir/ir.tree/tree-generator/src/org/jetbrains/kotlin/ir/generator/IrTree.kt

@@ -194,7 +194,6 @@ object IrTree : AbstractTreeBuilder() {
         +descriptor("ValueDescriptor")
         +symbol(valueSymbolType)
         +field("type", irTypeType)
-        +field("isAssignable", boolean, mutable = false)
     }
     val valueParameter: Element by element(Declaration) {
         needTransformMethod()
@@ -203,6 +202,7 @@ object IrTree : AbstractTreeBuilder() {
         parent(valueDeclaration)
 
         +descriptor("ParameterDescriptor")
+        +field("isAssignable", boolean, mutable = false)
         +symbol(valueParameterSymbolType)
         +field("index", int)
         +field("varargElementType", irTypeType, nullable = true)
@@ -597,16 +597,6 @@ object IrTree : AbstractTreeBuilder() {
         +field("isConst", boolean)
         +field("isLateinit", boolean)
         +field("initializer", expression, nullable = true)
-        +field("isAssignable", boolean, mutable = false) {
-            defaultValueInBase = "true"
-            withGetter = true
-            additionalImports.add(setValue)
-            kDoc = """
-            Variables are assignable by default. This means that they can be used in [${setValue.typeName}].
-            Variables are assigned in the IR even though they are not 'var' in the input. Hence
-            the separate assignability flag.
-            """.trimIndent()
-        }
     }
     val packageFragment: Element by element(Declaration) {
         ownsChildren = false