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/coercionToUnit.fir.kt.txt

@@ -14,14 +14,14 @@ fun test3() {
     val tmp0_safe_receiver: PrintStream? = #out
     when {
       EQEQ(arg0 = tmp0_safe_receiver, arg1 = null) -> null
-      else -> tmp0_safe_receiver /*as @FlexibleNullability PrintStream */.println(p0 = "Hello,")
+      else -> tmp0_safe_receiver.println(p0 = "Hello,")
     }
   } /*~> Unit */
   { // BLOCK
     val tmp1_safe_receiver: PrintStream? = #out
     when {
       EQEQ(arg0 = tmp1_safe_receiver, arg1 = null) -> null
-      else -> tmp1_safe_receiver /*as @FlexibleNullability PrintStream */.println(p0 = "world!")
+      else -> tmp1_safe_receiver.println(p0 = "world!")
     }
   } /*~> Unit */
 }