[ValueTracking] Recognize that and(x, add (x, -1)) clears the low bit

[ValueTracking] Recognize that and(x, add (x, -1)) clears the low bit

This is a cleaned up version of a patch by John Regehr with permission. Originally found via the souper tool.

If we add an odd number to x, then bitwise-and the result with x, we know that the low bit of the result must be zero. Either it was zero in x originally, or the add cleared it in the temporary value. As a result, one of the two values anded together must have the bit cleared.

Differential Revision: http://reviews.llvm.org/D14315

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@252629 91177308-0d34-0410-b5e6-96231b3b80d8