[mlir][spirv] Fix OOB error in TestModuleCombiner

[kuhar]

Summary

Also fix a typo in the surrounding code.

Fixes: https://github.com/llvm/llvm-project/issues/62318

[qedawkins]

LGTM

mlir/lib/Dialect/SPIRV/Linking/ModuleCombiner/ModuleCombiner.cpp

 #include "llvm/ADT/STLExtras.h"
 #include "llvm/ADT/StringExtras.h"
 #include "llvm/ADT/StringMap.h"
 
 using namespace mlir;
 
 static constexpr unsigned maxFreeID = 1 << 20;
 
-/// Returns an unsed symbol in `module` for `oldSymbolName` by trying numeric
+/// Returns an unused symbol in `module` for `oldSymbolName` by trying numeric
 /// suffix in `lastUsedID`.
 static StringAttr renameSymbol(StringRef oldSymName, unsigned &lastUsedID,
                                spirv::ModuleOp module) {
   SmallString<64> newSymName(oldSymName);
   newSymName.push_back('_');
 
   MLIRContext *ctx = module->getContext();
 

mlir/test/Dialect/SPIRV/Linking/ModuleCombiner/basic.mlir

 spirv.module Logical GLSL450 {
 }
 
 // expected-error @+1 {{input modules differ in addressing model, memory model, and/or VCE triple}}
 spirv.module Logical GLSL450 requires #spirv.vce<v1.0, [Shader], [SPV_KHR_storage_buffer_storage_class]> {
 }
 }
 
+// -----
+
+// No spir-v modules to combine.
+module {
+  func.func @foo(%arg0: i32) -> i32 {
+    return %arg0 : i32
+  }
+}

mlir/test/lib/Dialect/SPIRV/TestModuleCombiner.cpp

public:
   TestModuleCombinerPass() = default;
   TestModuleCombinerPass(const TestModuleCombinerPass &) {}
   void runOnOperation() override;
 };
 } // namespace
 
 void TestModuleCombinerPass::runOnOperation() {
   auto modules = llvm::to_vector<4>(getOperation().getOps<spirv::ModuleOp>());
+  if (modules.empty())
+    return;
 
   OpBuilder combinedModuleBuilder(modules[0]);
 
   auto listener = [](spirv::ModuleOp originalModule, StringRef oldSymbol,
                      StringRef newSymbol) {
     llvm::outs() << "[" << originalModule.getName() << "] " << oldSymbol
                  << " -> " << newSymbol << "\n";
   };