[turbofan] Implemented the optional Float32RoundDown operator.

src/x64/assembler-x64.cc

Index: src/x64/assembler-x64.cc
diff --git a/src/x64/assembler-x64.cc b/src/x64/assembler-x64.cc
index c8f99a11a64144fa38adb77b470a960c4619b047..9131d26f036fed1791002ab16f56a61e70cfca7b 100644
--- a/src/x64/assembler-x64.cc
+++ b/src/x64/assembler-x64.cc
@@ -3481,6 +3481,21 @@ void Assembler::cmpltsd(XMMRegister dst, XMMRegister src) {
 }
 
 
+void Assembler::roundss(XMMRegister dst, XMMRegister src, RoundingMode mode) {
+  DCHECK(!IsEnabled(AVX));
+  DCHECK(IsEnabled(SSE4_1));
+  EnsureSpace ensure_space(this);
+  emit(0x66);
+  emit_optional_rex_32(dst, src);
+  emit(0x0f);
+  emit(0x3a);
+  emit(0x0a);
+  emit_sse_operand(dst, src);
+  // Mask precision exception.
+  emit(static_cast<byte>(mode) | 0x8);
+}
+
+
 void Assembler::roundsd(XMMRegister dst, XMMRegister src, RoundingMode mode) {
   DCHECK(!IsEnabled(AVX));
   DCHECK(IsEnabled(SSE4_1));