This is a follow-up for r273544.
The end goal is to get rid of the isSwift / isCortexXY / isWhatever methods.
This commit also removes a command line flag that isn't used in any of the tests:
check-vmlx-hazards. It can be replaced easily with the mattr mechanism, since
this is now a subtarget feature.
I have mixed feelings about the FeatureExpandMLx. I'm not sure I understand the
history there. In particular, in the past MLx expansion was enabled for
subtargets with hasVFP2(), until r129775  switched from that to isCortexA9,
without too much justification.
In spite of that, the code performing MLx expansion still contains calls to
isSwift/isLikeA9, although the results of those are pretty clear given that
we're only enabling it for the A9. Should we try to enable it for other targets?
If so, do we want to have a feature for it (as it is in this patch), or should
we control it based on other features? Is FeatureHasVMLxHazards a good candidate
for this? Any thoughts appreciated, thanks.