Allow the type of unroll only when AllowRuntime is true.
Limit Count to DefaultRuntimeCount.
I'd prefer we used a different way to control this, like perhaps another option.
The way it is now overloads the documented meaning of "-unroll-runtime", which may be confusing and undesirable/restricting.
For example, if I disable -unroll-runtime to stop unrolling loops that don't have compile time constants (as it's documented to do), with this patch, I'll also stop some loops that DO have compile time constants from being unrolled.
We need to either re-purpose that option, or introduce a new one. I'd prefer the latter for minimal impact.