[XLA:GPU] Add check that computation is found in TritonFilecheckTest.

TritonFileCheckTest::CreateTritonIrAndFilecheck fails with an obscure segmentation fault in TritonFusionAnalysis::Execute if an incorrect name for the triton function is passed in. This makes it seem like the issue is around tiling analysis when it is just a test typo. This change adds a ret_check to clarify that the failure is from the call to GetComputationWithName.
PiperOrigin-RevId: 605007087

xla/service/gpu/ir_emitter_triton_test.cc

@@ -121,6 +121,7 @@ absl::StatusOr<bool> TritonFilecheckTest::CreateTritonIrAndFileCheck(
 
   auto* computation =
       verified_module->GetComputationWithName(triton_fusion_name);
+  TF_RET_CHECK(computation != nullptr);
   TF_ASSIGN_OR_RETURN(auto analysis,
                       TritonFusionAnalysis::Execute(*computation));