[PATCH] [AArch64] Select lower sub, abs pattern to sabd on AArch64

[Jyoti Allur]

A generic name for the testcase would be better.
Need to add checks for complete machine instructions in the test output rather than just pneumonic, since we have added different patterns to handle various types of data. 
Could you please modify to test these?
While above changes are needed, i would prefer we get a go from either of reviewers before committing.


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http://reviews.llvm.org/D6781

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