spirv-opt: DominatorAnalysis::Dominates does not handle OpLabel instructions

[Vasniktel]

The following test is supposed to fail.

TEST(SegfaultTest, Segfault) {
  std::string shader = R"(
               OpCapability Shader
          %1 = OpExtInstImport "GLSL.std.450"
               OpMemoryModel Logical GLSL450
               OpEntryPoint Fragment %4 "main"
               OpExecutionMode %4 OriginUpperLeft
               OpSource ESSL 320
          %2 = OpTypeVoid
          %3 = OpTypeFunction %2
          %4 = OpFunction %2 None %3
          %5 = OpLabel
          %6 = OpUndef %2
               OpReturn
               OpFunctionEnd
  )";

  const auto env = SPV_ENV_UNIVERSAL_1_3;
  const auto consumer = nullptr;
  const auto context = BuildModule(env, consumer, shader, kFuzzAssembleOption);
  ASSERT_TRUE(IsValid(env, context.get()));

  const auto& function = *context->module()->begin();
  auto* op_label = context->get_def_use_mgr()->GetDef(5);
  auto* op_undef = context->get_def_use_mgr()->GetDef(6);
  ASSERT_EXIT(context->GetDominatorAnalysis(&function)->Dominates(
                  op_label, op_undef),
              ::testing::KilledBySignal(SIGSEGV), ".*");
}

[Vasniktel]

@alan-baker @s-perron ping.