Make register allocator to disregard constant computation with no uses.

runtime/vm/assembler_x64.cc

Index: runtime/vm/assembler_x64.cc
diff --git a/runtime/vm/assembler_x64.cc b/runtime/vm/assembler_x64.cc
index 697060baedea88a92a805ee57f6612007748b705..783ee24db24aa5a31c02ce176a0d223b98df1a33 100644
--- a/runtime/vm/assembler_x64.cc
+++ b/runtime/vm/assembler_x64.cc
@@ -947,6 +947,13 @@ void Assembler::xorq(const Address& dst, Register src) {
 }
 
 
+void Assembler::xorq(Register dst, const Immediate& imm) {
+  AssemblerBuffer::EnsureCapacity ensured(&buffer_);
+  EmitRegisterREX(dst, REX_W);
+  EmitComplex(6, Operand(dst), imm);
+}
+
+
 void Assembler::addl(Register dst, Register src) {
   AssemblerBuffer::EnsureCapacity ensured(&buffer_);
   Operand operand(src);