X64: Implement fast charCodeAt.

src/x64/assembler-x64.h

Index: src/x64/assembler-x64.h
diff --git a/src/x64/assembler-x64.h b/src/x64/assembler-x64.h
index 4f482f55f3a722bf42f1ad7b24d1d08057f0feaa..015fa68237c801fe30166e3ebd408410674d9d5f 100644
--- a/src/x64/assembler-x64.h
+++ b/src/x64/assembler-x64.h
@@ -517,6 +517,8 @@ class Assembler : public Malloced {
   void movsxlq(Register dst, Register src);
   void movsxlq(Register dst, const Operand& src);
   void movzxbq(Register dst, const Operand& src);
+  void movzxbl(Register dst, const Operand& src);
+  void movzxwl(Register dst, const Operand& src);
 
   // New x64 instruction to load from an immediate 64-bit pointer into RAX.
   void load_rax(void* ptr, RelocInfo::Mode rmode);