fix Issue 12412. CTFE-able math

Issue:
https://issues.dlang.org/show_bug.cgi?id=12412

Make isNaN, isNormal, isSubnormal, isFinite, isInfinity CTFE-able.
@DmitryOlshansky are you sure? I thought `if (__ctfe)` blocks get removed before it gets to the stage were inlining is considered.