[turbofan] Implemented the optional Float32RoundDown operator.

src/ia32/assembler-ia32.h

Index: src/ia32/assembler-ia32.h
diff --git a/src/ia32/assembler-ia32.h b/src/ia32/assembler-ia32.h
index 15092951d77cd7cfe5f36092e51db45c216bea73..034029fa087e0507869b9fd58bc395186bda8a0b 100644
--- a/src/ia32/assembler-ia32.h
+++ b/src/ia32/assembler-ia32.h
@@ -989,6 +989,7 @@ class Assembler : public AssemblerBase {
   void ucomisd(XMMRegister dst, XMMRegister src) { ucomisd(dst, Operand(src)); }
   void ucomisd(XMMRegister dst, const Operand& src);
 
+  void roundss(XMMRegister dst, XMMRegister src, RoundingMode mode);
   void roundsd(XMMRegister dst, XMMRegister src, RoundingMode mode);
 
   void movmskpd(Register dst, XMMRegister src);