Implemented the Word64Popcnt TurboFan operator for x64.

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 5b0b9a04146904b787a5144d813922963448949d..d624447fb40c827c4bdcba014bcbcbe3eb5aaf67 100644
--- a/src/x64/macro-assembler-x64.cc
+++ b/src/x64/macro-assembler-x64.cc
@@ -3267,6 +3267,26 @@ void MacroAssembler::Popcntl(Register dst, const Operand& src) {
 }
 
 
+void MacroAssembler::Popcntq(Register dst, Register src) {
+  if (CpuFeatures::IsSupported(POPCNT)) {
+    CpuFeatureScope scope(this, POPCNT);
+    popcntq(dst, src);
+    return;
+  }
+  UNREACHABLE();
+}
+
+
+void MacroAssembler::Popcntq(Register dst, const Operand& src) {
+  if (CpuFeatures::IsSupported(POPCNT)) {
+    CpuFeatureScope scope(this, POPCNT);
+    popcntq(dst, src);
+    return;
+  }
+  UNREACHABLE();
+}
+
+
 void MacroAssembler::Pushad() {
   Push(rax);
   Push(rcx);