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/box/nullCheckOptimization/primitiveCheckWithSideEffect.kt

@@ -0,0 +1,23 @@
+var result = "fail 1"
+
+fun withSideEffect() : Int {
+    result = "OK"
+    return 42
+}
+
+fun box(): String {
+    if (withSideEffect() == null) {
+        return "fail 2"
+    }
+
+    if (result != "OK") {
+        return "fail 3"
+    }
+
+    result = "fail 4"
+    if (withSideEffect() != null) {
+        return result
+    }
+
+    return result
+}