Add pshufw instruction, fix inconsistencies with pextrw instruction.

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 d3f25137994a2343b1d2d36a2c789ee251b973a0..6a28b51fc89e1e9d1b507f113c2b662ab0689e3d 100644
--- a/src/x64/assembler-x64.h
+++ b/src/x64/assembler-x64.h
@@ -889,6 +889,9 @@ class Assembler : public AssemblerBase {
   void ud2();
   void setcc(Condition cc, Register reg);
 
+  void pshufw(XMMRegister dst, XMMRegister src, uint8_t shuffle);
+  void pshufw(XMMRegister dst, const Operand& src, uint8_t shuffle);
+
   // Label operations & relative jumps (PPUM Appendix D)
   //
   // Takes a branch opcode (cc) and a label (L) and generates