Add roundsd instruction to ia32 and use it in optimized Math.floor.

src/ia32/assembler-ia32.h

Index: src/ia32/assembler-ia32.h
===================================================================
--- src/ia32/assembler-ia32.h	(revision 8916)
+++ src/ia32/assembler-ia32.h	(working copy)
@@ -941,6 +941,16 @@
   void andpd(XMMRegister dst, XMMRegister src);
 
   void ucomisd(XMMRegister dst, XMMRegister src);
+
+  enum RoundingMode {
+    kRoundToNearest = 0x0,
+    kRoundDown      = 0x1,
+    kRoundUp        = 0x2,
+    kRoundToZero    = 0x3
+  };
+
+  void roundsd(XMMRegister dst, XMMRegister src, RoundingMode mode);
+
   void movmskpd(Register dst, XMMRegister src);
 
   void cmpltsd(XMMRegister dst, XMMRegister src);