That isn't an advantage, IMHO. rustc generates LLVM IR, which allows it to take advantage of essentially all the same optimization passes that clang does. Generating LLVM IR instead of C has some advantages—debug information can be made more precise, precise aliasing metadata for optimization can be added to the output, accurate garbage collection is possible, there's no need to go through the C parser and semantic analyzer, etc.

Examples of features that rustc uses today for optimization beyond those available in C are the "nonnull" and "dereferenceable" attributes (along with some custom optimization passes to make non-null pointer optimizations stronger than what LLVM provides out of the box). An example of a feature beyond C that it could use, but does not today, is custom type-based alias analysis.