Stage2 can't negate a NaN at runtime

[Vexu]

In the AIR negation is implemented as 0 - val which means that a NaN cannot be negated using the - operator. I'm not sure how desirable negative NaNs are but the behavior should be consistent at comptime and runtime.

test {
    std.debug.print("{d}\n", .{-math.nan(f32)}); // nan
    std.debug.print("{d}\n", .{-comptime math.nan(f32)}); // -nan
}

[andrewrk]

stage1 lowers to

  %2 = fneg float %1

stage2 lowers to

  %2 = fsub float 0.000000e+00, %1

stage1 is correct