JVM IR: Don't produce CHECKCASTs on null constants (KT-36650)

compiler/testData/codegen/bytecodeText/checkcast/kt15411.kt

@@ -1,7 +1,3 @@
-// IGNORE_BACKEND: JVM_IR
-// TODO KT-36650 Don't generate CHECKCAST on null values in JVM_IR
-// KT-15411 Unnecessary CHECKCAST bytecode when dealing with null
-
 fun test1(): String? {
     return null
 }