JVM: do not put public field access into a single expression block

It affects the `is IrGetField` check in
TypeOperatorLowering.computeNotNullAssertionText, which leads to missing
NPE messages when accessing backing fields of public properties.

 #KT-64615

compiler/ir/backend.jvm/lower/src/org/jetbrains/kotlin/backend/jvm/lower/JvmOptimizationLowering.kt

@@ -136,9 +136,12 @@ class JvmOptimizationLowering(val context: JvmBackendContext) : FileLoweringPass
                 } else {
                     +irGetField(receiver.takeUnless { backingField.isStatic }, backingField)
                 }
-            }
+            }.unwrapSingleExpressionBlock()
         }
 
+        private fun IrExpression.unwrapSingleExpressionBlock(): IrExpression =
+            (this as? IrBlock)?.statements?.singleOrNull() as? IrExpression ?: this
+
         override fun visitWhen(expression: IrWhen, data: IrDeclaration?): IrExpression {
             val isCompilerGenerated = expression.origin == null
             expression.transformChildren(this, data)