[JVM IR] Fix issue where fields are not being set to their default

values within initializer blocks.

The issue occurs in code like this:
```
class C {
  var b = true
  init {
    b = false   // Missing PUTFIELD for this statement
  }
}
```

Added a new statement origin for field initialization (at declaration)
instead of relying on `origin == null` in ExpressionCodegen to determine
whether to generate the initializations.

This was unintentionally broken in
d68a1898d0.

compiler/ir/backend.common/src/org/jetbrains/kotlin/backend/common/lower/InitializersLowering.kt

@@ -9,10 +9,7 @@ import org.jetbrains.kotlin.backend.common.*
 import org.jetbrains.kotlin.ir.IrElement
 import org.jetbrains.kotlin.ir.IrStatement
 import org.jetbrains.kotlin.ir.declarations.*
-import org.jetbrains.kotlin.ir.expressions.IrBody
-import org.jetbrains.kotlin.ir.expressions.IrExpression
-import org.jetbrains.kotlin.ir.expressions.IrInstanceInitializerCall
-import org.jetbrains.kotlin.ir.expressions.IrStatementOriginImpl
+import org.jetbrains.kotlin.ir.expressions.*
 import org.jetbrains.kotlin.ir.expressions.impl.IrBlockImpl
 import org.jetbrains.kotlin.ir.expressions.impl.IrExpressionBodyImpl
 import org.jetbrains.kotlin.ir.expressions.impl.IrGetValueImpl
@@ -79,7 +76,15 @@ abstract class InitializersLoweringBase(open val context: CommonBackendContext)
                 IrGetValueImpl(startOffset, endOffset, irClass.thisReceiver!!.type, irClass.thisReceiver!!.symbol)
             else
                 null
-            IrSetFieldImpl(startOffset, endOffset, declaration.symbol, receiver, expression, context.irBuiltIns.unitType)
+            IrSetFieldImpl(
+                startOffset,
+                endOffset,
+                declaration.symbol,
+                receiver,
+                expression,
+                context.irBuiltIns.unitType,
+                IrStatementOrigin.INITIALIZE_FIELD
+            )
         }
 
     private fun handleAnonymousInitializer(declaration: IrAnonymousInitializer): IrStatement =

compiler/ir/backend.jvm/src/org/jetbrains/kotlin/backend/jvm/codegen/ExpressionCodegen.kt

@@ -565,9 +565,7 @@ class ExpressionCodegen(
         // Do not add redundant field initializers that initialize to default values.
         val inPrimaryConstructor = irFunction is IrConstructor && irFunction.isPrimary
         val inClassInit = irFunction.origin == JvmLoweredDeclarationOrigin.CLASS_STATIC_INITIALIZER
-        // "expression.origin == null" means that the field is initialized when it is declared,
-        // i.e., not in an initializer block or constructor body.
-        val isFieldInitializer = expression.origin == null
+        val isFieldInitializer = expression.origin == IrStatementOrigin.INITIALIZE_FIELD
         val skip = (inPrimaryConstructor || inClassInit) && isFieldInitializer && expressionValue is IrConst<*> &&
                 isDefaultValueForType(expression.symbol.owner.type.asmType, expressionValue.value)
         return if (skip) defaultValue(expression.type) else super.visitSetField(expression, data)

compiler/ir/ir.tree/src/org/jetbrains/kotlin/ir/expressions/IrStatementOrigin.kt

@@ -91,6 +91,7 @@ interface IrStatementOrigin {
     object OBJECT_LITERAL : IrStatementOriginImpl("OBJECT_LITERAL")
 
     object INITIALIZE_PROPERTY_FROM_PARAMETER : IrStatementOriginImpl("INITIALIZE_PROPERTY_FROM_PARAMETER")
+    object INITIALIZE_FIELD : IrStatementOriginImpl("INITIALIZE_FIELD")
 
     object PROPERTY_REFERENCE_FOR_DELEGATE : IrStatementOriginImpl("PROPERTY_REFERENCE_FOR_DELEGATE")