[turbofan] Implemented the optional Float32RoundDown 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 5575f1bfd88ad62b71f08d8f3562c62a4d8bfa8d..cf29f39f10d90ae1370b826635385ddfb5ca7923 100644
--- a/src/compiler/mips/instruction-selector-mips.cc
+++ b/src/compiler/mips/instruction-selector-mips.cc
@@ -647,6 +647,9 @@ void InstructionSelector::VisitFloat64Sqrt(Node* node) {
 }
 
 
+void InstructionSelector::VisitFloat32RoundDown(Node* node) { UNREACHABLE(); }
+
+
 void InstructionSelector::VisitFloat64RoundDown(Node* node) {
   VisitRR(this, kMipsFloat64RoundDown, node);
 }