Hi,
For the following test-case:

int16x8_t foo(int16_t x, int16_t y)
{
  return (int16x8_t) { x, y, x, y, x, y, x, y };
}

Code gen at -O3:
foo:
        dup    v0.8h, w0
        ins     v0.h[1], w1
        ins     v0.h[3], w1
        ins     v0.h[5], w1
        ins     v0.h[7], w1
        ret

For 16 elements, it results in 8 ins instructions which might not be
optimal perhaps.
I guess, the above code-gen would be equivalent to the following ?
dup v0.8h, w0
dup v1.8h, w1
zip1 v0.8h, v0.8h, v1.8h

I have attached patch to do the same, if number of elements >= 8,
which should be possibly better compared to current code-gen ?
Patch passes bootstrap+test on aarch64-linux-gnu.
Does the patch look OK ?

Thanks,
Prathamesh