Summary:
@public
`-ffast-math` does not have measurable performance benefits.
By using `NaN` for *undefined* values again, we can squeeze `YGFloatOptional` into 32 bits.
This will also enable us to store `YGValue` (or a variant of it) in 32 bits.
Reviewed By: SidharthGuglani
Differential Revision: D13119110
fbshipit-source-id: 4e6964240bf74ebc22d8783107b89d536a1a0842