KT-36813 Generate optimizable null checks in JVM_IR

compiler/testData/codegen/box/nullCheckOptimization/varModifiedAfterCheck.kt

@@ -0,0 +1,59 @@
+// TARGET_BACKEND: JVM
+// FILE: varModifiedAfterCheck.kt
+fun implicitCheck(s: String) {}
+
+fun test1() {
+    var a = J.foo()
+    val b = a
+    a = J.bar()
+    if (b != null) {
+        if (a != null) {
+            throw AssertionError("a: $a")
+        }
+    }
+}
+
+fun test2() {
+    var a = J.foo()
+    val b = a
+    if (b != null) {
+        a = J.bar()
+        if (a != null) {
+            throw AssertionError("a: $a")
+        }
+    }
+}
+
+fun test3() {
+    var a = J.foo()
+    val b = a
+    a = J.bar()
+    implicitCheck(b)
+    if (a != null) {
+        throw AssertionError("a: $a")
+    }
+}
+
+fun test4() {
+    var a = J.foo()
+    val b = a
+    implicitCheck(b)
+    a = J.bar()
+    if (a != null) {
+        throw AssertionError("a: $a")
+    }
+}
+
+fun box(): String {
+    test1()
+    test2()
+    test3()
+    test4()
+    return "OK"
+}
+
+// FILE: J.java
+public class J {
+    public static String foo() { return ""; }
+    public static String bar() { return null; }
+}