Automatic optimization of C / C ++ code

I would like to optimize the C / C ++ code and get an optimized version in C or intermediate representation (for example, LLVM IR), without actually compiling the code, mainly due to the lack of dependencies. In other words, I am looking for a way to see the optimization results on specific pieces of C code, assuming that any compiler will not compile the code due to missing libraries and dependencies.

Is it possible to use optimizations built into open source C / C ++ compilers (like GCC and Clang) without compiling the code? I'm mainly interested in the flow control graph (CFG) of functions after optimization. I am fully aware of the fact that this will not be entirely possible and will almost certainly give different results compared to when dependencies are present (due to nesting, for example).