[firtool] Enable CSE on RTL

The CSE pass has been fixed upstream to not crash on graph regions with
use-before-def's in the IR.

tools/firtool/firtool.cpp

@@ -200,9 +200,7 @@ processBuffer(std::unique_ptr<llvm::MemoryBuffer> ownedBuffer,
     if (!disableOptimization) {
       auto &modulePM = pm.nest<rtl::RTLModuleOp>();
       modulePM.addPass(sv::createRTLCleanupPass());
-      // FIXME: Disabled because CSE crashes on graph regions in some cases.
-      // Issue #813: https://github.com/llvm/circt/issues/813
-      // modulePM.addPass(createCSEPass());
+      modulePM.addPass(createCSEPass());
       modulePM.addPass(createCanonicalizerPass());
     }
   }