Implement constant folding in the IR backend for JVM

The newly added pass folds the set of constant functions of the
current backend, plus IrBuiltIns.

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

@@ -0,0 +1,21 @@
+class A
+
+fun foo(x: Any?) {}
+
+fun box(u: Int) {
+    val x: Int? = 1
+    x!!
+
+    val z: Int? = if (u == 1) x else null
+    z!!
+
+    foo(1 as java.lang.Integer)
+
+    val y: Any? = if (u == 1) x else A()
+    y!!
+}
+
+// 0 IFNULL
+// 1 IFNONNULL
+// 1 throwNpe
+// 0 ATHROW