Tests updated to explicitly use fast-isel at -O0 instead of implicitly.
We should properly fall back to selection-dag in the cases where we don't support some feature.
This change also allows an explicit -fast-isel option to override an implicitly enabled global-isel. Otherwise -fast-isel would have no effect at -O0.
I would rather have another target hook for the abort case.
Same thing as what we have for GISel enabled (option takes precedence if set, otherwise we look at what the target wants)