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/testData/diagnostics/testsWithStdLib/contracts/smartcasts/operatorsTests/equalsWithNullableBoolean.fir.kt

@@ -28,7 +28,7 @@ fun equalsFalse(x: Any?) {
         x.<!UNRESOLVED_REFERENCE!>length<!>
     }
     else {
-        x.length
+        x.<!UNRESOLVED_REFERENCE!>length<!>
     }
 }
 
@@ -52,7 +52,7 @@ fun notEqualsTrue(x: Any?) {
 
 fun notEqualsFalse(x: Any?) {
     if (safeIsString(x) != false) {
-        x.length
+        x.<!UNRESOLVED_REFERENCE!>length<!>
     }
     else {
         x.<!UNRESOLVED_REFERENCE!>length<!>
@@ -66,4 +66,4 @@ fun notEqualsNull(x: Any?) {
     else {
         x.<!UNRESOLVED_REFERENCE!>length<!>
     }
-}
+}