FIR DFA: smartcast variable to Nothing? on null assignment

In order to make resolution still work for members not available from
`Nothing`, we track the type without `Nothing?` and use that for
resolution instead.

compiler/testData/diagnostics/tests/BinaryCallsOnNullableValues.fir.kt

@@ -5,24 +5,24 @@ class A() {
 fun f(): Unit {
     var x: Int? = 1
     x = null
-    x <!UNSAFE_OPERATOR_CALL!>+<!> 1
+    x + 1
     x <!UNSAFE_INFIX_CALL!>plus<!> 1
     x <!UNSAFE_OPERATOR_CALL!><<!> 1
-    x <!UNSAFE_OPERATOR_CALL!>+=<!> 1
+    <!ASSIGNMENT_TYPE_MISMATCH!>x += 1<!>
 
     x == 1
     x != 1
 
     <!EQUALITY_NOT_APPLICABLE!>A() == 1<!>
 
-    <!EQUALITY_NOT_APPLICABLE!>x === "1"<!>
-    <!EQUALITY_NOT_APPLICABLE!>x !== "1"<!>
+    x === "1"
+    x !== "1"
 
     x === 1
     x !== 1
 
-    x<!UNSAFE_OPERATOR_CALL!>..<!>2
-    <!ARGUMENT_TYPE_MISMATCH!>x<!> in 1..2
+    x..2
+    x in 1..2
 
     val y : Boolean? = true
     false || <!CONDITION_TYPE_MISMATCH!>y<!>