ir: avoid dependency on isnan
ir: avoid dependency on isnan

there's a simple way to check for nan that does not need this header.
hryx on IRC reported that Linux Mint based on ubuntu 16.04, kernel
4.15.0, x86_64 did not have the isnan function.

andrewrk/zig,andrewrk/zig,zig-lang/zig,zig-lang/zig,zig-lang/zig,raulgrell/zig,raulgrell/zig,raulgrell/zig,raulgrell/zig,raulgrell/zig