Use if constexpr

[cyyever]

Description

Add "if constexpr" support.

[rwgk]

I'd rather not make these changes, but I'm open to other opinions.

if constexpr is a game changer as an alternative to SFINAE acrobatics.

But AFAIK it does very little or nothing for optimization purposes.

My opinion in this particular case: some extra code noise, no clear benefit.