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

src/ia32/assembler-ia32.cc

Index: src/ia32/assembler-ia32.cc
===================================================================
--- src/ia32/assembler-ia32.cc	(revision 8916)
+++ src/ia32/assembler-ia32.cc	(working copy)
@@ -1957,6 +1957,18 @@
 }
 
 
+void Assembler::roundsd(XMMRegister dst, XMMRegister src, RoundingMode mode) {
+  ASSERT(CpuFeatures::IsEnabled(SSE4_1));
+  EnsureSpace ensure_space(this);
+  EMIT(0x66);
+  EMIT(0x0F);
+  EMIT(0x3A);
+  EMIT(0x0B);
+  emit_sse_operand(dst, src);
+  // Mask precision exeption.
+  EMIT(static_cast<byte>(mode) | 0x8);
+}
+
 void Assembler::movmskpd(Register dst, XMMRegister src) {
   ASSERT(CpuFeatures::IsEnabled(SSE2));
   EnsureSpace ensure_space(this);