JVM_IR: Optimize null checks.

Introduce lowering to remove null checks for primitive type
expressions and replace them with true/false. Side-effects
are preserved.

Generate ifnull/ifnonnull instructions for null checks instead
of materializing a null literal for an equality check.

compiler/testData/codegen/bytecodeText/boxingOptimization/kt7224.kt

@@ -1,4 +1,3 @@
-// IGNORE_BACKEND: JVM_IR
 fun box(): String {
     230?.hashCode()
 

compiler/testData/codegen/bytecodeText/boxingOptimization/simpleUninitializedMerge.kt

@@ -1,4 +1,3 @@
-// IGNORE_BACKEND: JVM_IR
 fun box(): String {
     var result = 0
     if (1 == 1) {

compiler/testData/codegen/bytecodeText/boxingOptimization/unsafeRemoving.kt

@@ -1,4 +1,3 @@
-// IGNORE_BACKEND: JVM_IR
 fun returningBoxed() : Int? = 1
 fun acceptingBoxed(x : Int?) : Int ? = x
 

compiler/testData/codegen/bytecodeText/conditions/negatedNullCompare.kt

@@ -1,4 +1,3 @@
-// IGNORE_BACKEND: JVM_IR
 fun main(p: String?) {
     if (!(p == null)) {
         "then"

compiler/testData/codegen/bytecodeText/conditions/nullCompare.kt

@@ -1,4 +1,3 @@
-// IGNORE_BACKEND: JVM_IR
 fun main(p: String?) {
     if (p == null) {
         "then"

compiler/testData/codegen/bytecodeText/deadCodeElimination/boxing.kt

@@ -1,4 +1,3 @@
-// IGNORE_BACKEND: JVM_IR
 class A
 
 fun foo(x: Any?) {}

compiler/testData/codegen/bytecodeText/deadCodeElimination/literal.kt

@@ -1,4 +1,3 @@
-// IGNORE_BACKEND: JVM_IR
 fun box() {
     val x: Any? = "abc"
     val y: Any? = if (1 == 1) x else "cde"

compiler/testData/codegen/bytecodeText/deadCodeElimination/simpleConstructor.kt

@@ -1,4 +1,3 @@
-// IGNORE_BACKEND: JVM_IR
 class A
 fun box() {
     val x: A? = A()

compiler/testData/codegen/bytecodeText/deadCodeElimination/simpleConstructorNotRedundant.kt

@@ -1,4 +1,3 @@
-// IGNORE_BACKEND: JVM_IR
 class A
 fun box() {
     val x: A? = A()

compiler/testData/codegen/bytecodeText/ieee754/smartCastsForDouble.kt

@@ -1,5 +1,4 @@
 // !LANGUAGE: +ProperIeee754Comparisons
-// IGNORE_BACKEND: JVM_IR
 fun equals5(a: Any?, b: Any?) = if (a is Double && b is Double?) a == b else null!!
 
 fun equals6(a: Any?, b: Any?) = if (a is Double? && b is Double) a == b else null!!

compiler/testData/codegen/bytecodeText/ieee754/smartCastsForFloat.kt

@@ -1,5 +1,4 @@
 // !LANGUAGE: +ProperIeee754Comparisons
-// IGNORE_BACKEND: JVM_IR
 fun equals5(a: Any?, b: Any?) = if (a is Float && b is Float?) a == b else null!!
 
 fun equals6(a: Any?, b: Any?) = if (a is Float? && b is Float) a == b else null!!

compiler/testData/codegen/bytecodeText/intConstantNullable.kt

@@ -1,4 +1,3 @@
-// IGNORE_BACKEND: JVM_IR
 val a : Int? = 10
 
 fun foo() = a!!.toString()

compiler/testData/codegen/bytecodeText/nullCheckOptimization/alreadyCheckedForNull.kt

@@ -1,4 +1,3 @@
-// IGNORE_BACKEND: JVM_IR
 fun test(x: String?) {
     if (x == null) return
 

compiler/testData/codegen/bytecodeText/nullCheckOptimization/ifNullEqualsNull.kt

@@ -1,4 +1,3 @@
-// IGNORE_BACKEND: JVM_IR
 fun test1() {
     val a = null
 

compiler/testData/codegen/bytecodeText/nullCheckOptimization/ifUnitEqualsNull.kt

@@ -1,4 +1,3 @@
-// IGNORE_BACKEND: JVM_IR
 fun test1() {
     val a = Unit
 

compiler/testData/codegen/bytecodeText/nullCheckOptimization/kt12839.kt

@@ -1,4 +1,3 @@
-// IGNORE_BACKEND: JVM_IR
 fun test() {
     val value = System.getProperty("key")
     if (value != null) {

compiler/testData/codegen/bytecodeText/nullCheckOptimization/localLateinit/checkedAlways.kt

@@ -1,4 +1,3 @@
-// IGNORE_BACKEND: JVM_IR
 fun almostAlwaysTrue() = true
 
 fun runNoInline(f: () -> Unit) = f()

compiler/testData/codegen/bytecodeText/nullCheckOptimization/localLateinit/initialized.kt

@@ -1,3 +1,4 @@
+// IGNORE_BACKEND: JVM_IR
 fun test() {
     lateinit var z: String
     run {

compiler/testData/codegen/bytecodeText/nullCheckOptimization/primitiveCheck.kt

@@ -0,0 +1,18 @@
+fun f() : Int {
+    return 42
+}
+
+fun box(): String {
+    if (f() == null) {
+        return "fail 1"
+    }
+
+    if (f() != null) {
+        return "OK"
+    }
+
+    return "fail 2"
+}
+
+// 0 IF
+// 0 ACONST_NULL

compiler/testData/codegen/bytecodeText/when/kt18818.kt

@@ -1,4 +1,3 @@
-// IGNORE_BACKEND: JVM_IR
 fun findUserId(username: String): Long? = null
 
 fun main(args: Array<String>) {