use explicit simd for iszero check on partials

[KristofferC]

Now:

julia> code_native(ForwardDiff.iszero_tuple, Tuple{NTuple{4, Float64}}; debuginfo=:none)
        .section        __TEXT,__text,regular,pure_instructions
        vxorpd  %xmm0, %xmm0, %xmm0
        vcmpneqpd       (%rdi), %ymm0, %ymm0
        vmovmskpd       %ymm0, %eax
        testb   %al, %al
        sete    %al
        vzeroupper
        retq
        nopw    %cs:(%rax,%rax)

On master:

julia> code_native(ForwardDiff.iszero_tuple, Tuple{NTuple{4, Float64}}; debuginfo=:none)
        .section        __TEXT,__text,regular,pure_instructions
        vmovsd  (%rdi), %xmm1                   ## xmm1 = mem[0],zero
        vxorpd  %xmm0, %xmm0, %xmm0
        vucomisd        %xmm0, %xmm1
        jne     L46
        jp      L46
        vmovsd  8(%rdi), %xmm1                  ## xmm1 = mem[0],zero
        vucomisd        %xmm0, %xmm1
        jne     L46
        jp      L46
        vmovsd  16(%rdi), %xmm1                 ## xmm1 = mem[0],zero
        vxorpd  %xmm0, %xmm0, %xmm0
        vucomisd        %xmm0, %xmm1
        jne     L46
        jnp     L49
L46:
        xorl    %eax, %eax
        retq
L49:
        vcmpeqsd        24(%rdi), %xmm0, %xmm0
        vmovq   %xmm0, %rax
        andl    $1, %eax
        retq

[codecov-commenter]

Codecov Report

Merging #559 (4469639) into master (d033d2a) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #559      +/-   ##
==========================================
+ Coverage   84.95%   84.97%   +0.01%     
==========================================
  Files           9        9              
  Lines         844      845       +1     
==========================================
+ Hits          717      718       +1     
  Misses        127      127              

Impacted Files	Coverage Δ
src/partials.jl	84.48% <100.00%> (+0.13%)	⬆️

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