(PopCount8 x) => (POPCNTL (MOVBQZX <typ.UInt32> x))

(Sqrt ...) => (SQRTSD ...)
(Sqrt32 ...) => (SQRTSS ...)

(RoundToEven x) => (ROUNDSD [0] x)
(Floor x)       => (ROUNDSD [1] x)
(Ceil x)        => (ROUNDSD [2] x)
(Trunc x)       => (ROUNDSD [3] x)

(FMA x y z) => (VFMADD231SD z x y)

// Lowering extension
// Note: we always extend to 64 bits even though some ops don't need that many result bits.