[builtins] Introduce proper Float64Atan and Float64Atan2 operators.

src/compiler/js-builtin-reducer.h

Index: src/compiler/js-builtin-reducer.h
diff --git a/src/compiler/js-builtin-reducer.h b/src/compiler/js-builtin-reducer.h
index db308f79a8dded61acf59f0e4513c8358ad0515b..1e61928138d1a8b90aeeebac34f0a997943c7094 100644
--- a/src/compiler/js-builtin-reducer.h
+++ b/src/compiler/js-builtin-reducer.h
@@ -37,6 +37,8 @@ class JSBuiltinReducer final : public AdvancedReducer {
   Reduction ReduceMathClz32(Node* node);
   Reduction ReduceMathFloor(Node* node);
   Reduction ReduceMathFround(Node* node);
+  Reduction ReduceMathAtan(Node* node);
+  Reduction ReduceMathAtan2(Node* node);
   Reduction ReduceMathLog(Node* node);
   Reduction ReduceMathLog1p(Node* node);
   Reduction ReduceMathRound(Node* node);