Merging r155818:

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r155818 | baldrick | 2012-04-30 04:56:58 -0700 (Mon, 30 Apr 2012) | 3 lines

Just mark the sign bit as known zero, rather than any other irrelevant bits
known zero in the LHS.  Fixes PR12541.

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git-svn-id: https://llvm.org/svn/llvm-project/llvm/branches/release_31@155827 91177308-0d34-0410-b5e6-96231b3b80d8