FIR DFA: don't assume != true/false => == false/true

This also fixes some returnsNotNull contracts because the old code added
an implication that `== true` => `!= null` then promptly removed any
statement that this could've affected if the argument was a synthetic
variable.

^KT-26612 tag fixed-in-k2

compiler/tests-spec/testData/diagnostics/notLinked/contracts/analysis/common/neg/1.fir.kt

@@ -66,7 +66,7 @@ fun case_2(value_1: Int?, value_2: Int?, value_3: Any?) {
             println(value_2)
         }
         null -> {
-            println(value_3?.xor(true))
+            <!OVERLOAD_RESOLUTION_AMBIGUITY!>println<!>(value_3?.<!UNRESOLVED_REFERENCE!>xor<!>(true))
             println(<!UNINITIALIZED_VARIABLE!>value_4<!>)
             println(value_1)
             println(value_2)

compiler/tests-spec/testData/diagnostics/notLinked/contracts/analysis/common/pos/1.fir.kt

@@ -61,7 +61,7 @@ fun case_2(value_1: Int?, value_2: Int?, value_3: Any?) {
             println(value_2)
         }
         null -> {
-            <!OVERLOAD_RESOLUTION_AMBIGUITY!>println<!>(value_3?.<!UNRESOLVED_REFERENCE!>xor<!>(true))
+            println(value_3?.xor(true))
             println(value_4)
             println(value_1)
             println(value_2)