[turbofan] Added the RoundInt64ToFloat64 instruction to TurboFan.

src/x64/macro-assembler-x64.cc

Index: src/x64/macro-assembler-x64.cc
diff --git a/src/x64/macro-assembler-x64.cc b/src/x64/macro-assembler-x64.cc
index 48ad4c347e7d715f2bdf35b07e4777638738bed0..d10e45e68d717219089cc343fc1059447e83a4d8 100644
--- a/src/x64/macro-assembler-x64.cc
+++ b/src/x64/macro-assembler-x64.cc
@@ -863,6 +863,18 @@ void MacroAssembler::Cvtqsi2sd(XMMRegister dst, Register src) {
 }
 
 
+void MacroAssembler::Cvtqsi2sd(XMMRegister dst, const Operand& src) {
+  if (CpuFeatures::IsSupported(AVX)) {
+    CpuFeatureScope scope(this, AVX);
+    vxorpd(dst, dst, dst);
+    vcvtqsi2sd(dst, dst, src);
+  } else {
+    xorpd(dst, dst);
+    cvtqsi2sd(dst, src);
+  }
+}
+
+
 void MacroAssembler::Cvtsd2si(Register dst, XMMRegister src) {
   if (CpuFeatures::IsSupported(AVX)) {
     CpuFeatureScope scope(this, AVX);