message: Log an admonition to ERROR instead of CHECK-failing when XLA GPU is not -O2
  Being able to set the opt level is useful when debugging.
diff: @@ -394,8 +394,16 @@ StatusOr<string> CompileModuleToPtx(llvm::Module* module,
  int32 opt_level =
  hlo_module_config.debug_options().xla_backend_optimization_level();
  - CHECK_GE(opt_level, 2)
  - << "The XLA GPU backend doesn't support unoptimized code generation";
  + if (opt_level < 2) {
  + LOG(ERROR) << std::string(80, '*');
  + LOG(ERROR) << "The XLA GPU backend doesn't support unoptimized code "
  + "generation but ";
  + LOG(ERROR) << "--xla_backend_optimization_level is set to " << opt_level
  + << "!";
  + LOG(ERROR) << "(Supported configuration is "
  + "--xla_backend_optimization_level >= 2.)";
  + LOG(ERROR) << std::string(80, '*');
  + }
  AddOptimizationPasses(opt_level,
  /*size_level=*/0, target_machine.get(), &module_passes,
  