[x86] fix initialization of PredictableSelectIsExpensive

This is effectively NFC because Atom is the only in-order x86 subtarget currently,
but the predicate would have become wrong if any other in-order CPU came along.

See related discussion in:
http://reviews.llvm.org/D16836



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@261275 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed