Generalise VEC_DUPLICATE folding for variable-length vectors

Generalise VEC_DUPLICATE folding for variable-length vectors

This patch uses the constant vector encoding scheme to handle
more cases of a VEC_DUPLICATE of another vector.  Duplicating
any fixed-length vector is fine, and duplicating a variable-length
vector is OK as long as that vector is also a duplicate of a
fixed-length sequence.

Other cases fell through to:

  if (VECTOR_MODE_P (mode) && GET_CODE (op) == CONST_VECTOR)

which was only expecting to deal with elementwise operations.

2019-07-29  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	* simplify-rtx.c (simplify_const_unary_operation): Fold a
	VEC_DUPLICATE of a fixed-length vector even if the result
	is variable-length.  Likewise fold a duplicate of a
	variable-length vector if the variable-length vector is
	itself a duplicate of a fixed-length sequence.
	(test_vector_ops_duplicate): Test more cases.

From-SVN: r273868