[FIR] Minor. Add transformResult to FirWhenBranch

compiler/fir/tree/src/org/jetbrains/kotlin/fir/expressions/FirWhenBranch.kt

@@ -29,4 +29,6 @@ interface FirWhenBranch : FirElement {
     }
 
     fun <D> transformCondition(transformer: FirTransformer<D>, data: D): FirWhenBranch
+
+    fun <D> transformResult(transformer: FirTransformer<D>, data: D): FirWhenBranch
 }

compiler/fir/tree/src/org/jetbrains/kotlin/fir/expressions/impl/FirWhenBranchImpl.kt

@@ -21,7 +21,7 @@ class FirWhenBranchImpl(
 ) : FirAbstractElement(psi), FirWhenBranch {
     override fun <D> transformChildren(transformer: FirTransformer<D>, data: D): FirElement {
         transformCondition(transformer, data)
-        result = result.transformSingle(transformer, data)
+        transformResult(transformer, data)
         return this
     }
 
@@ -29,4 +29,9 @@ class FirWhenBranchImpl(
         condition = condition.transformSingle(transformer, data)
         return this
     }
+
+    override fun <D> transformResult(transformer: FirTransformer<D>, data: D): FirWhenBranch {
+        result = result.transformSingle(transformer, data)
+        return this
+    }
 }