Make register allocator to disregard constant computation with no uses.

runtime/vm/assembler_ia32.cc

Index: runtime/vm/assembler_ia32.cc
diff --git a/runtime/vm/assembler_ia32.cc b/runtime/vm/assembler_ia32.cc
index 6627cd649251cd813b286b9aeb8eaf3ddd8351a4..dbba71ba15573a8d6a528d576e198817b8a2834a 100644
--- a/runtime/vm/assembler_ia32.cc
+++ b/runtime/vm/assembler_ia32.cc
@@ -885,6 +885,12 @@ void Assembler::xorl(Register dst, Register src) {
 }
 
 
+void Assembler::xorl(Register dst, const Immediate& imm) {
+  AssemblerBuffer::EnsureCapacity ensured(&buffer_);
+  EmitComplex(6, Operand(dst), imm);
+}
+
+
 void Assembler::addl(Register reg, const Immediate& imm) {
   AssemblerBuffer::EnsureCapacity ensured(&buffer_);
   EmitComplex(0, Operand(reg), imm);