X64: Use roundsd for DoMathFloor.

src/x64/disasm-x64.cc

Index: src/x64/disasm-x64.cc
diff --git a/src/x64/disasm-x64.cc b/src/x64/disasm-x64.cc
index 2b7b7b7d334e35a8d16bc583d9d32165924fec24..1d708aee959d8c1a991f496a4a89aba814807024 100644
--- a/src/x64/disasm-x64.cc
+++ b/src/x64/disasm-x64.cc
@@ -1021,6 +1021,13 @@ int DisassemblerX64::TwoByteOpcodeInstruction(byte* data) {
         current += PrintRightOperand(current);
         AppendToBuffer(", %s, %d", NameOfCPURegister(regop), (*current) & 3);
         current += 1;
+      } else if (third_byte == 0x0b) {
+        get_modrm(*current, &mod, &regop, &rm);
+         // roundsd xmm, xmm/m64, imm8
+        AppendToBuffer("roundsd %s, ", NameOfCPURegister(regop));
+        current += PrintRightOperand(current);
+        AppendToBuffer(", %d", (*current) & 3);
+        current += 1;
       } else {
         UnimplementedInstruction();
       }