Implemented the Word64Popcnt TurboFan operator for x64.

src/compiler/x64/code-generator-x64.cc

Index: src/compiler/x64/code-generator-x64.cc
diff --git a/src/compiler/x64/code-generator-x64.cc b/src/compiler/x64/code-generator-x64.cc
index ccafb3cbed2e26f1821697b8360302982054dfbc..095e3a5d18e38e78d41fcb2d87a40b872f1d72cf 100644
--- a/src/compiler/x64/code-generator-x64.cc
+++ b/src/compiler/x64/code-generator-x64.cc
@@ -796,6 +796,13 @@ void CodeGenerator::AssembleArchInstruction(Instruction* instr) {
         __ Tzcntl(i.OutputRegister(), i.InputOperand(0));
       }
       break;
+    case kX64Popcnt:
+      if (instr->InputAt(0)->IsRegister()) {
+        __ Popcntq(i.OutputRegister(), i.InputRegister(0));
+      } else {
+        __ Popcntq(i.OutputRegister(), i.InputOperand(0));
+      }
+      break;
     case kX64Popcnt32:
       if (instr->InputAt(0)->IsRegister()) {
         __ Popcntl(i.OutputRegister(), i.InputRegister(0));