[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 17321ce0e49cec7b7059e59498632705835e4331..84542b60991f9c9323a6babeb3a74f1ca673358c 100644
--- a/src/compiler/x64/code-generator-x64.cc
+++ b/src/compiler/x64/code-generator-x64.cc
@@ -937,6 +937,13 @@ 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;