[turbofan] Introduce TruncateTaggedToWord32 simplified operator.

src/compiler/x64/instruction-selector-x64.cc

Index: src/compiler/x64/instruction-selector-x64.cc
diff --git a/src/compiler/x64/instruction-selector-x64.cc b/src/compiler/x64/instruction-selector-x64.cc
index dba3d9b2309b5085e623125173bde7e28b8fdeaf..b1af3e56d89e106ac159e6cc623643e771b99770 100644
--- a/src/compiler/x64/instruction-selector-x64.cc
+++ b/src/compiler/x64/instruction-selector-x64.cc
@@ -1141,15 +1141,8 @@ void InstructionSelector::VisitTruncateFloat64ToFloat32(Node* node) {
   VisitRO(this, node, kSSEFloat64ToFloat32);
 }
 
-
-void InstructionSelector::VisitTruncateFloat64ToInt32(Node* node) {
-  switch (TruncationModeOf(node->op())) {
-    case TruncationMode::kJavaScript:
-      return VisitRR(this, node, kArchTruncateDoubleToI);
-    case TruncationMode::kRoundToZero:
-      return VisitRO(this, node, kSSEFloat64ToInt32);
-  }
-  UNREACHABLE();
+void InstructionSelector::VisitTruncateFloat64ToWord32(Node* node) {
+  VisitRR(this, node, kArchTruncateDoubleToI);
 }
 
 
@@ -1175,6 +1168,9 @@ void InstructionSelector::VisitTruncateInt64ToInt32(Node* node) {
   Emit(kX64Movl, g.DefineAsRegister(node), g.Use(value));
 }
 
+void InstructionSelector::VisitRoundFloat64ToInt32(Node* node) {
+  VisitRO(this, node, kSSEFloat64ToInt32);
+}
 
 void InstructionSelector::VisitRoundInt32ToFloat32(Node* node) {
   X64OperandGenerator g(this);