[FIR] Improve performance when tracking local variable assignment

During data-flow analysis, mutable local variables need not have their
assignments tracked before they can be smart-cast. This is in case a
local variable is mutated within a loop or captured and mutated within a
closure. There was a significant performance problem with this analysis
which could cause exponential degredation based on the number of when
branches or catch blocks.

^KT-69723 Fixed

compiler/fir/resolve/src/org/jetbrains/kotlin/fir/resolve/dfa/FirLocalVariableAssignmentAnalyzer.kt

@@ -338,8 +338,8 @@ internal class FirLocalVariableAssignmentAnalyzer {
                 return property in assignments
             }
 
-            fun add(property: FirProperty, assignment: Assignment) {
-                assignments.getOrPut(property) { mutableSetOf() }.add(assignment)
+            fun add(property: FirProperty, assignment: Assignment): Boolean {
+                return assignments.getOrPut(property) { mutableSetOf() }.add(assignment)
             }
 
             fun copy(): VariableAssignments {
@@ -348,19 +348,20 @@ internal class FirLocalVariableAssignmentAnalyzer {
                 return copy
             }
 
-            fun merge(other: VariableAssignments?) {
-                if (other == null) return
+            fun merge(other: VariableAssignments?): Boolean {
+                if (other == null || other.assignments.isEmpty()) return false
+
+                var modified = false
                 for ((property, values) in other.assignments) {
-                    assignments.getOrPut(property) { mutableSetOf() }.addAll(values)
+                    modified = modified or assignments.getOrPut(property) { mutableSetOf() }.addAll(values)
                 }
+                return modified
             }
 
             fun retain(properties: Set<FirProperty>) {
                 assignments.keys.retainAll(properties)
             }
 
-            fun isEmpty(): Boolean = assignments.isEmpty()
-
             fun getAssignedProperties(): Set<FirPropertySymbol> {
                 return assignments.entries
                     // TODO(KT-57563): Operator assignments should be treated just like any other assignment.
@@ -518,14 +519,12 @@ internal class FirLocalVariableAssignmentAnalyzer {
             }
 
             private fun MiniFlow.recordAssignment(property: FirProperty, assignment: Assignment) {
-                assignedLater.add(property, assignment)
+                if (!assignedLater.add(property, assignment)) return // Parents do not need to be updated if this flow is not updated.
                 parents.forEach { it.recordAssignment(property, assignment) }
             }
 
             private fun MiniFlow.recordAssignments(properties: VariableAssignments) {
-                if (properties.isEmpty()) return
-
-                assignedLater.merge(properties)
+                if (!assignedLater.merge(properties)) return // Parents do not need to be updated if this flow is not updated.
                 parents.forEach { it.recordAssignments(properties) }
             }