JVM_IR more aggressive if-null expressions fusion

Assume that changing a return type from T to T?
is not a binary-compatible change.

compiler/testData/codegen/box/boxingOptimization/safeCallWithElvisAndEnhancedNullability.kt

@@ -0,0 +1,50 @@
+// TARGET_BACKEND: JVM
+// WITH_RUNTIME
+
+// FILE: safeCallWithElvisAndEnhancedNullability.kt
+
+import kotlin.test.assertEquals
+
+fun check(a : A?) : Int {
+    return a?.y?.x ?: (a?.x ?: 3)
+}
+
+fun checkLeftAssoc(a : A?) : Int {
+    return (a?.y?.x ?: a?.x) ?: 3
+}
+
+fun box() : String {
+    val a1 = A(2, A(1, null))
+    val a2 = A(2, null)
+    val a3 = null
+
+    assertEquals(1, check(a1))
+    assertEquals(2, check(a2))
+    assertEquals(3, check(a3))
+
+    assertEquals(1, checkLeftAssoc(a1))
+    assertEquals(2, checkLeftAssoc(a2))
+    assertEquals(3, checkLeftAssoc(a3))
+
+    return "OK"
+}
+
+// FILE: A.java
+import org.jetbrains.annotations.NotNull;
+import org.jetbrains.annotations.Nullable;
+
+public class A {
+    private Integer x;
+    private A y;
+
+    public A(Integer x, A y) {
+        this.x = x;
+        this.y = y;
+    }
+
+    @NotNull
+    public Integer getX() { return x; }
+
+    @Nullable
+    public A getY() { return y; }
+}