[builtins] Introduce proper Float64Atan and Float64Atan2 operators.

src/compiler/simplified-lowering.cc

Index: src/compiler/simplified-lowering.cc
diff --git a/src/compiler/simplified-lowering.cc b/src/compiler/simplified-lowering.cc
index 45babfbd2d7e61d86ab2624362ea2c2d3456aada..937d56a6e76b4547a4b1f0bbef392f2165b6d517 100644
--- a/src/compiler/simplified-lowering.cc
+++ b/src/compiler/simplified-lowering.cc
@@ -1393,6 +1393,13 @@ class RepresentationSelector {
         if (lower()) DeferReplacement(node, lowering->Float64Floor(node));
         return;
       }
+      case IrOpcode::kNumberAtan2: {
+        VisitBinop(node, UseInfo::TruncatingFloat64(),
+                   MachineRepresentation::kFloat64);
+        if (lower()) NodeProperties::ChangeOp(node, Float64Op(node));
+        return;
+      }
+      case IrOpcode::kNumberAtan:
       case IrOpcode::kNumberLog:
       case IrOpcode::kNumberLog1p: {
         VisitUnop(node, UseInfo::TruncatingFloat64(),