fix Issue 12412. CTFE-able math

Issue:
https://issues.dlang.org/show_bug.cgi?id=12412

Make isNaN, isNormal, isSubnormal, isFinite, isInfinity CTFE-able.
At least it did screw NVRO. About inlining I'm not entirily sure, needs testing, but it seems to leave in about the same pass.