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/constants/string.kt

@@ -1,4 +1,3 @@
-// IGNORE_BACKEND: JVM_IR
 fun box(): String {
     return "O" + "K".toString() + 1.toLong()
 }