[turbofan] Implemented the optional Float32RoundDown operator.

src/compiler/x64/code-generator-x64.cc

Index: src/compiler/x64/code-generator-x64.cc
diff --git a/src/compiler/x64/code-generator-x64.cc b/src/compiler/x64/code-generator-x64.cc
index 8fac455aa35d7901eaeb551c330c4181a7666034..aa0be1386276912cc434aea565131f0216d85f89 100644
--- a/src/compiler/x64/code-generator-x64.cc
+++ b/src/compiler/x64/code-generator-x64.cc
@@ -929,6 +929,13 @@ void CodeGenerator::AssembleArchInstruction(Instruction* instr) {
     case kSSEFloat32ToFloat64:
       ASSEMBLE_SSE_UNOP(Cvtss2sd);
       break;
+    case kSSEFloat32Round: {
+      CpuFeatureScope sse_scope(masm(), SSE4_1);
+      RoundingMode const mode =
+          static_cast<RoundingMode>(MiscField::decode(instr->opcode()));
+      __ Roundss(i.OutputDoubleRegister(), i.InputDoubleRegister(0), mode);
+      break;
+    }
     case kSSEFloat64Cmp:
       ASSEMBLE_SSE_BINOP(Ucomisd);
       break;