[Issue 20200] Make std.math.isPowerOf2 work in CTFE

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=20200

--- Comment #1 from Nathan S. <n8sh.secondary at hotmail.com> ---
isPowerOf2 already worked in CTFE for integers but not floating point numbers.

--