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/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