[polly] r177831 - RegisterPasses: Improve comments

[Sebastian Pop]

Tobias Grosser wrote:
>  
> -  // Polly is only enabled at -O3
> +  // We only run Polly at optimization level '-O3'.
> +  //
> +  // This is to ensure that scalar overhead that may be introduced by Polly is
> +  // properly cleaned up by LLVM later on. We may reinvestigate this decision
> +  // later on.
>    if (Builder.OptLevel != 3) {
>      errs() << "Polly should only be run with -O3. Disabling Polly.\n";

I don't think that polly should enforce this with an error: it's not erroneous
to want to try the code generated by polly at a different opt level than -O3.
What about removing this check and allow polly to execute at any opt level?