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/fir/semantics/src/org/jetbrains/kotlin/fir/resolve/dfa/model.kt

@@ -19,31 +19,22 @@ class MutableTypeStatement(
     fun copy(): MutableTypeStatement = MutableTypeStatement(variable, LinkedHashSet(exactType))
 }
 
-fun Implication.invertCondition(): Implication = Implication(condition.invert(), effect)
-
 // --------------------------------------- Aliases ---------------------------------------
 
 typealias TypeStatements = Map<RealVariable, TypeStatement>
 
 // --------------------------------------- DSL ---------------------------------------
 
-infix fun DataFlowVariable.eq(constant: Boolean?): OperationStatement {
-    val condition = when (constant) {
-        true -> Operation.EqTrue
-        false -> Operation.EqFalse
-        null -> Operation.EqNull
-    }
-    return OperationStatement(this, condition)
-}
+infix fun DataFlowVariable.eq(constant: Boolean): OperationStatement =
+    OperationStatement(this, if (constant) Operation.EqTrue else Operation.EqFalse)
 
-infix fun DataFlowVariable.notEq(constant: Boolean?): OperationStatement {
-    val condition = when (constant) {
-        true -> Operation.EqFalse
-        false -> Operation.EqTrue
-        null -> Operation.NotEqNull
-    }
-    return OperationStatement(this, condition)
-}
+@Suppress("UNUSED_PARAMETER")
+infix fun DataFlowVariable.eq(constant: Nothing?): OperationStatement =
+    OperationStatement(this, Operation.EqNull)
+
+@Suppress("UNUSED_PARAMETER")
+infix fun DataFlowVariable.notEq(constant: Nothing?): OperationStatement =
+    OperationStatement(this, Operation.NotEqNull)
 
 infix fun OperationStatement.implies(effect: Statement): Implication = Implication(this, effect)
 

compiler/fir/semantics/src/org/jetbrains/kotlin/fir/resolve/dfa/statements.kt

@@ -19,10 +19,6 @@ sealed class Statement {
  * d == False
  */
 data class OperationStatement(override val variable: DataFlowVariable, val operation: Operation) : Statement() {
-    fun invert(): OperationStatement {
-        return OperationStatement(variable, operation.invert())
-    }
-
     override fun toString(): String {
         return "$variable $operation"
     }
@@ -55,13 +51,6 @@ class Implication(
 enum class Operation {
     EqTrue, EqFalse, EqNull, NotEqNull;
 
-    fun invert(): Operation = when (this) {
-        EqTrue -> EqFalse
-        EqFalse -> EqTrue
-        EqNull -> NotEqNull
-        NotEqNull -> EqNull
-    }
-
     fun valueIfKnown(given: Operation): Boolean? = when (this) {
         EqTrue, EqFalse -> if (given == NotEqNull) null else given == this
         EqNull -> given == EqNull