[turbofan] Implemented the optional Float32RoundDown operator.

src/x64/macro-assembler-x64.cc

Index: src/x64/macro-assembler-x64.cc
diff --git a/src/x64/macro-assembler-x64.cc b/src/x64/macro-assembler-x64.cc
index fbbc0688e1ce2453a35c66d797d221059f867d6f..dae2e74e2ee8c14bde960743bbb31eac42fb53f1 100644
--- a/src/x64/macro-assembler-x64.cc
+++ b/src/x64/macro-assembler-x64.cc
@@ -2764,6 +2764,17 @@ void MacroAssembler::Movmskpd(Register dst, XMMRegister src) {
 }
 
 
+void MacroAssembler::Roundss(XMMRegister dst, XMMRegister src,
+                             RoundingMode mode) {
+  if (CpuFeatures::IsSupported(AVX)) {
+    CpuFeatureScope scope(this, AVX);
+    vroundss(dst, dst, src, mode);
+  } else {
+    roundss(dst, src, mode);
+  }
+}
+
+
 void MacroAssembler::Roundsd(XMMRegister dst, XMMRegister src,
                              RoundingMode mode) {
   if (CpuFeatures::IsSupported(AVX)) {