Simplify vec_duplicates of vec_duplicates

r17-899-g52d5a8870d2108 added a pattern with a nested vec_duplicate,
but I think those should be folded down to a single vec_duplicate.

The later:

  if (VECTOR_MODE_P (mode)
      && vec_duplicate_p (op, &elt)
      && code != VEC_DUPLICATE)

handles only scalar elements, but this rule applies to vec_duplicates
of vectors as well.

A bit of scripting suggests that this was the only use of nested
vec_duplicates in the md files.

This is tested by gcc.target/aarch64/sve/vec_init_5.c.

gcc/
	* simplify-rtx.cc (simplify_context::simplify_unary_operation_1):
	Simplify vec_duplicates of vec_duplicates.
	* config/aarch64/aarch64-sve.md
	(*aarch64_vec_duplicate_subvector<vconsv><vconq><mode>): Remove
	nested vec_duplicate.