Hi Richard,
While digging thru aarch64_expand_vector_init, I noticed it gives
priority to loading a constant first:
 /* Initialise a vector which is part-variable.  We want to first try
     to build those lanes which are constant in the most efficient way we
     can.  */

which results in suboptimal code-gen for following case:
int16x8_t f_s16(int16_t x)
{
  return (int16x8_t) { x, x, x, x, x, x, x, 1 };
}

code-gen trunk:
f_s16:
        movi    v0.8h, 0x1
        ins     v0.h[0], w0
        ins     v0.h[1], w0
        ins     v0.h[2], w0
        ins     v0.h[3], w0
        ins     v0.h[4], w0
        ins     v0.h[5], w0
        ins     v0.h[6], w0
        ret

The attached patch tweaks the following condition:
if (n_var == n_elts && n_elts <= 16)
  {
    ...
  }

to pass if maxv >= 80% of n_elts, with 80% being an
arbitrary "high enough" threshold. The intent is to dup
the most repeating variable if it it's repetition
is "high enough" and insert constants which should be "better" than
loading constant first and inserting variables like in the above case.

Alternatively, I suppose we can remove threshold and for constants,
generate both sequences and check which one is more
efficient ?

code-gen with patch:
f_s16:
        dup     v0.8h, w0
        movi    v1.4h, 0x1
        ins     v0.h[7], v1.h[0]
        ret

The patch is lightly tested to verify that vec[t]-init-*.c tests pass
with bootstrap+test
in progress.
Does this look OK ?

Thanks,
Prathamesh