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/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