FIR2IR: Do not add implicit casts for types with different nullability

For smart casts, elvises, etc., there are no implicit casts in psi2fir
in changed test data

compiler/testData/ir/irText/expressions/when.fir.kt.txt

@@ -24,9 +24,9 @@ fun testWithSubject(x: Any?): String {
 fun test(x: Any?): String {
   return when {
     EQEQ(arg0 = x, arg1 = null) -> "null"
-    EQEQ(arg0 = x /*as Any */, arg1 = A) -> "A"
-    x /*as Any */ is String -> "String"
-    x /*as Any */ !is Number -> "!Number"
+    EQEQ(arg0 = x, arg1 = A) -> "A"
+    x is String -> "String"
+    x !is Number -> "!Number"
     setOf<Nothing>().contains<Number>(element = x /*as Number */) -> "nothingness?"
     else -> "something"
   }