[X86] Add test cases for the patterns used by __builtin_ffs.
[X86] Add test cases for the patterns used by __builtin_ffs.

We previously had tests for "__builtin_ffs + 5", but the SelectinoDAG without an extra addition came out slightly different.

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@338612 91177308-0d34-0410-b5e6-96231b3b80d8

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