Add fast 64-bit bitwise negation to the IA32 optimizing compiler.

runtime/vm/assembler_ia32.cc

Index: runtime/vm/assembler_ia32.cc
===================================================================
--- runtime/vm/assembler_ia32.cc	(revision 13214)
+++ runtime/vm/assembler_ia32.cc	(working copy)
@@ -797,6 +797,15 @@
 }
 
 
+void Assembler::pxor(XmmRegister dst, XmmRegister src) {
+  AssemblerBuffer::EnsureCapacity ensured(&buffer_);
+  EmitUint8(0x66);
+  EmitUint8(0x0F);
+  EmitUint8(0xEF);
+  EmitXmmRegisterOperand(dst, src);
+}
+
+
 void Assembler::fldl(const Address& src) {
   AssemblerBuffer::EnsureCapacity ensured(&buffer_);
   EmitUint8(0xDD);