Fix ClassCastException with nullable chars

#KT-4210 Fixed
 #KT-4098 Fixed
 #KT-4251 Fixed

compiler/testData/codegen/box/primitiveTypes/kt4098.kt

@@ -0,0 +1,10 @@
+fun box(): String {
+    val c = '0': Char?
+    c!!.toInt()
+
+    "123456"?.get(0)!!.toInt()
+
+    "123456"!!.get(0).toInt()
+
+    return "OK"
+}

compiler/testData/codegen/box/primitiveTypes/kt4210.kt

@@ -0,0 +1,16 @@
+fun box(): String {
+    val s: String? = "abc"
+    val c = s?.get(0)!! - 'b'
+    if (c != -1) return "Fail c: $c"
+
+    val d = 'b' - s?.get(2)!!
+    if (d != -1) return "Fail d: $d"
+
+    val e = s?.get(2)!! - s?.get(0)!!
+    if (e != 2) return "Fail e: $e"
+
+    val f = s?.get(2)!!.minus(s?.get(0)!!)
+    if (f != 2) return "Fail f: $f"
+
+    return "OK"
+}

compiler/testData/codegen/box/primitiveTypes/kt4251.kt

@@ -0,0 +1,5 @@
+fun box(): String {
+    val a: Char? = 'a'
+    val result = a!! < 'b'
+    return if (result) "OK" else "Fail"
+}

compiler/testData/codegen/box/primitiveTypes/nullableCharBoolean.kt

@@ -0,0 +1,9 @@
+fun box(): String {
+    val c: Char? = 'a'
+    if (c!! - 'a' != 0) return "Fail c"
+
+    val b: Boolean? = false
+    if (b!!) return "Fail b"
+
+    return "OK"
+}