JVM_IR KT-44993 preserve inner expression type when fusing if-null

compiler/testData/codegen/box/regressions/kt44993.kt

@@ -0,0 +1,15 @@
+// TARGET_BACKEND: JVM
+// WITH_RUNTIME
+// FILE: kt44993.kt
+fun box(): String =
+    f(KotlinBox(JavaBox()))
+
+fun f(r: KotlinBox<JavaBox>): String =
+    r?.data?.element!!
+
+class KotlinBox<T>(@JvmField val data: T?)
+
+// FILE: JavaBox.java
+public class JavaBox {
+    public final String element = "OK";
+}