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/safeAssignment.fir.txt

@@ -54,6 +54,5 @@ FILE fqName:<root> fileName:/safeAssignment.kt
             BRANCH
               if: CONST Boolean type=kotlin.Boolean value=true
               then: CALL 'public final fun <set-x> (<set-?>: kotlin.Int): kotlin.Unit declared in <root>.C' type=kotlin.Unit origin=EQ
-                $this: TYPE_OP type=<root>.C origin=IMPLICIT_CAST typeOperand=<root>.C
-                  GET_VAR 'val tmp_0: <root>.C? [val] declared in <root>.test' type=<root>.C? origin=null
+                $this: GET_VAR 'val tmp_0: <root>.C? [val] declared in <root>.test' type=<root>.C? origin=null
                 <set-?>: CONST Int type=kotlin.Int value=42