ValueTracking: Teach isKnownToBeAPowerOfTwo about (ADD X, (XOR X, Y)) where X is a power of two

This allows us to simplify urem instructions involving the add+xor to
turn into simpler math.
Prevent '\b' from backing up into invalid memory.  Fixes http://llvm.org/bugs/show_bug.cgi?id=16240.  Sorry, I can not think of a good test case for this one, except by running valgrind as reported in the bug.