[builtins] Introduce proper Float64Atan and Float64Atan2 operators.

src/compiler/machine-operator.h

Index: src/compiler/machine-operator.h
diff --git a/src/compiler/machine-operator.h b/src/compiler/machine-operator.h
index a8a07fa695e890f9a7e5e56ded1d433c9018c911..a199fd51824eef4c0096879b9cd9a6c47716eefd 100644
--- a/src/compiler/machine-operator.h
+++ b/src/compiler/machine-operator.h
@@ -368,6 +368,10 @@ class MachineOperatorBuilder final : public ZoneObject {
   const OptionalOperator Float32Neg();
   const OptionalOperator Float64Neg();
 
+  // Floating point trigonometric functions (double-precision).
+  const Operator* Float64Atan();
+  const Operator* Float64Atan2();
+
   // Floating point logarithm (double-precision).
   const Operator* Float64Log();
   const Operator* Float64Log1p();