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/testData/codegen/box/javaInterop/notNullAssertions/expressionAssertionMessages/backingField.kt

@@ -0,0 +1,29 @@
+// !LANGUAGE: +NoSourceCodeInNotNullAssertionExceptions
+// TARGET_BACKEND: JVM
+// FILE: test.kt
+
+val publicProperty = J().s()
+private val privateProperty = J().s()
+
+fun f(x: String) = "Fail 1"
+
+fun box(): String {
+    try {
+        f(publicProperty)
+    } catch (e: NullPointerException) {
+        if (e.message != "publicProperty must not be null") return "Fail 2: ${e.message}"
+    }
+
+    try {
+        f(privateProperty)
+    } catch (e: NullPointerException) {
+        if (e.message != "privateProperty must not be null") return "Fail 3: ${e.message}"
+    }
+
+    return "OK"
+}
+
+// FILE: J.java
+public class J {
+    public final String s() { return null; }
+}