[x86] fix initialization of PredictableSelectIsExpensive
[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: 0ff597fd157e6f4fc38580e8d64ab130330d2411@261275 91177308-0d34-0410-b5e6-96231b3b80d8

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