Small improvements to:

Make Simplify treat bitwise and as Mod if the and is with a constant
of form (2^n -1).

Make bounds reasoning handle shifts and bitwise_and the same way
Simplify does.

Small performance improvement to is_const_power_of_two.
Change test for overflow to not use undefined behavior (relative to
C++ standard) for signed integer types.
Merge pull request #584 from halide/bitwise_improvements

Small improvements for idiomatic use of shift and bitwise and (&) operators.