When returning a 'cannot match due to missing CPU features' error code,
if there are multiple potential matches with different feature sets,
return the smallest set of missing features from the alternatives as
that's most likely to be the one that's desired.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@158673 91177308-0d34-0410-b5e6-96231b3b80d8