builder: add support for -opt=0

This optimization level wasn't working before because some passes expect
some globals to be cleaned up afterwards. Cleaning these globals is
easy, just add the pass necessary for it. This shouldn't reduce the
usefulness of the -opt=0 build flag as most optimizations are still
skipped.

transform/optimizer.go

@@ -111,6 +111,12 @@ func Optimize(mod llvm.Module, config *compileopts.Config, optLevel, sizeLevel i
 		if len(errs) > 0 {
 			return errs
 		}
+
+		// Clean up some leftover symbols of the previous transformations.
+		goPasses := llvm.NewPassManager()
+		defer goPasses.Dispose()
+		goPasses.AddGlobalDCEPass()
+		goPasses.Run(mod)
 	}
 
 	// Lower async implementations.