FIR DFA: revalidate reassigned variables after loops

If a certain type statement is true on loop entry and all continue
paths, then it is also true on exit if the condition did not reassign
the variable.

^KT-7676 tag fixed-in-k2

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

@@ -21,7 +21,6 @@ abstract class LogicSystem<FLOW : Flow>(protected val context: ConeInferenceCont
     abstract fun addImplication(flow: FLOW, implication: Implication)
     abstract fun addLocalVariableAlias(flow: FLOW, alias: RealVariable, underlyingVariable: RealVariable)
     abstract fun recordNewAssignment(flow: FLOW, variable: RealVariable, index: Int)
-    abstract fun removeAllAboutVariable(flow: FLOW, variable: RealVariable)
     abstract fun copyAllInformation(from: FLOW, to: FLOW)
     abstract fun isSameValueIn(a: FLOW, b: FLOW, variable: RealVariable): Boolean
 
@@ -91,9 +90,9 @@ abstract class LogicSystem<FLOW : Flow>(protected val context: ConeInferenceCont
         exactType += other.exactType
     }
 
-    protected fun and(statements: Collection<TypeStatement>): TypeStatement? =
+    fun and(statements: Collection<TypeStatement>): TypeStatement? =
         statements.singleOrNew { statements.flatMapTo(mutableSetOf()) { it.exactType } }
 
-    protected fun or(statements: Collection<TypeStatement>): TypeStatement? =
+    fun or(statements: Collection<TypeStatement>): TypeStatement? =
         statements.singleOrNew { unifyTypes(statements.map { it.exactType })?.let { mutableSetOf(it) } ?: mutableSetOf() }
 }

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

@@ -165,10 +165,6 @@ abstract class PersistentLogicSystem(context: ConeInferenceContext) : LogicSyste
         flow.addAliases(persistentSetOf(alias), flow.unwrapVariable(underlyingVariable))
     }
 
-    override fun removeAllAboutVariable(flow: PersistentFlow, variable: RealVariable) {
-        flow.replaceVariable(variable, null)
-    }
-
     private fun PersistentFlow.replaceVariable(variable: RealVariable, replacement: RealVariable?) {
         val original = directAliasMap[variable]
         if (original != null) {
@@ -320,7 +316,7 @@ abstract class PersistentLogicSystem(context: ConeInferenceContext) : LogicSyste
     }
 
     override fun recordNewAssignment(flow: PersistentFlow, variable: RealVariable, index: Int) {
-        removeAllAboutVariable(flow, variable)
+        flow.replaceVariable(variable, null)
         flow.assignmentIndex = flow.assignmentIndex.put(variable, index)
     }