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/safeCallWithElvisMultipleFiles.kt

@@ -0,0 +1,33 @@
+// WITH_RUNTIME
+
+// MODULE: lib
+// FILE: a.kt
+class A(val x : Int, val y : A?)
+
+// MODULE: main(lib)
+// FILE: safeCallWithElvisMultipleFiles.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"
+}