RFA: Small PATCH to add pow2p_hwi to hwint.h

[Joseph Myers]

On Thu, 8 Sep 2016, Jason Merrill wrote:

> Various places in GCC use negate, bit-and and compare to test whether
> an integer is a power of 2, but I think it would be clearer for this
> test to be wrapped in a function.

(x & -x) == x is also true for 0.  Whatever the correct function semantics 
for 0, the comment needs to reflect them.

-- 
Joseph S. Myers
joseph@codesourcery.com