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.kt.txt

@@ -17,7 +17,7 @@ fun test(nc: C?) {
     val tmp0_safe_receiver: C? = nc
     when {
       EQEQ(arg0 = tmp0_safe_receiver, arg1 = null) -> null
-      else -> tmp0_safe_receiver /*as C */.<set-x>(<set-?> = 42)
+      else -> tmp0_safe_receiver.<set-x>(<set-?> = 42)
     }
   } /*~> Unit */
 }