[PATCH] Remove spurious mask operations from AArch64 add->compares on 16 and 8 bit values

[Louis Gerbarg]

This is the patch I discussed a while back in http://lists.cs.uiuc.edu/pipermail/llvmdev/2014-July/074444.html. It eliminates spurious ands on 8 and 16 bit operations that have been promoted to 32 bit that have no material impact on these test cases they are fed into. It remove them in several cases that have come up in testing. It is entirely possible there are missed cases due to a slightly differently configured DAG, but adding those should be simple if we have examples of them. This patch has been tested against LNT on AArch64 hardware and includes tests that were randomly generated using on device data as well as ones derives from actual bugs.

Louis
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