[turbofan] Introduce TruncateTaggedToWord32 simplified operator.

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

Index: src/compiler/mips/instruction-selector-mips.cc
diff --git a/src/compiler/mips/instruction-selector-mips.cc b/src/compiler/mips/instruction-selector-mips.cc
index e78d9f4c74b9c5d4ec09079714cf505d203876f0..3e0e3d8599ddf3432a186e2a66f5d568809e2b70 100644
--- a/src/compiler/mips/instruction-selector-mips.cc
+++ b/src/compiler/mips/instruction-selector-mips.cc
@@ -719,17 +719,13 @@ void InstructionSelector::VisitTruncateFloat64ToFloat32(Node* node) {
   VisitRR(this, kMipsCvtSD, node);
 }
 
-
-void InstructionSelector::VisitTruncateFloat64ToInt32(Node* node) {
-  switch (TruncationModeOf(node->op())) {
-    case TruncationMode::kJavaScript:
-      return VisitRR(this, kArchTruncateDoubleToI, node);
-    case TruncationMode::kRoundToZero:
-      return VisitRR(this, kMipsTruncWD, node);
-  }
-  UNREACHABLE();
+void InstructionSelector::VisitTruncateFloat64ToWord32(Node* node) {
+  VisitRR(this, kArchTruncateDoubleToI, node);
 }
 
+void InstructionSelector::VisitRoundFloat64ToInt32(Node* node) {
+  VisitRR(this, kMipsTruncWD, node);
+}
 
 void InstructionSelector::VisitBitcastFloat32ToInt32(Node* node) {
   VisitRR(this, kMipsFloat64ExtractLowWord32, node);