use muladd in horner macro

[simonbyrne]

Polynomials should now be able to take advantage of fma operations when available (cf. #9840).

cc: @stevengj, @eschnett

[ivarne]

Test failiure:

ERROR: LoadError: LoadError: test error in expression: @evalpoly 2 3 4 5 6 == 3 + 2 * (4 + 2 * (5 + 2 * 6)) == @evalpoly 2 + 0im 3 4 5 6
muladd not defined for Int64
while loading math.jl, in expression starting on line 327
while loading /tmp/julia/share/julia/test/runtests.jl, in expression starting on line 42

Should we have a default fallback for muladd(::Number, ::Number, ::Number)?

[simonbyrne]

Yes, there should be a fallback.

[eschnett]

The line

muladd{T<:Number}(x::T, y::T, z::T) = no_op_err("muladd", T)

in promotion.cl should instead read

muladd{T<:Number}(x::T, y::T, z::T) = x*y+z

[eschnett]

I have opened #9882 that also adds test cases for muladd.

[stevengj]

Shouldn't we also use it in @evalpoly, which also has multiply-accumulate operations on real operands? (Except that it has $ai * tt + $b, $b + r*$ai, and $(esc(p[i])) - s * $ai, where the last is an fnma operation. Just muladd is not enough, as I've pointed out several times since #6330.)

[simonbyrne]

Ah, good point: I forgot about @evalpoly.