[mlir] Bug fix. Stream must outlive the pass manager.

[rdzhabarov]

Summary

Bug fix. Stream must outlive the pass manager.

mlir/lib/Dialect/GPU/Transforms/SerializeToBlob.cpp

gpu::SerializeToBlobPass::translateToISA(llvm::Module &llvmModule,
   llvmModule.setDataLayout(targetMachine.createDataLayout());
 
   if (failed(optimizeLlvm(llvmModule, targetMachine)))
     return llvm::None;
 
   std::string targetISA;
   llvm::raw_string_ostream stream(targetISA);
 
-  llvm::legacy::PassManager codegenPasses;
-
   { // Drop pstream after this to prevent the ISA from being stuck buffering
     llvm::buffer_ostream pstream(stream);
+    llvm::legacy::PassManager codegenPasses;
+
     if (targetMachine.addPassesToEmitFile(codegenPasses, pstream, nullptr,
                                           llvm::CGFT_AssemblyFile))
       return llvm::None;
 
     codegenPasses.run(llvmModule);
   }
   return stream.str();
 }