[AArch64] Use vec_perm_indices helper routines

[AArch64] Use vec_perm_indices helper routines

This patch makes the AArch64 vec_perm_const code use the new
vec_perm_indices routines, instead of checking each element individually.
This means that they extend naturally to variable-length vectors.

Also, aarch64_evpc_dup was the only function that generated rtl when
testing_p is true, and that looked accidental.  The patch adds the
missing check and then replaces the gen_rtx_REG/start_sequence/
end_sequence stuff with an assert that no rtl is generated.

2018-01-09  Richard Sandiford  <richard.sandiford@linaro.org>

gcc/
	* config/aarch64/aarch64.c (aarch64_evpc_trn): Use d.perm.series_p
	instead of checking each element individually.
	(aarch64_evpc_uzp): Likewise.
	(aarch64_evpc_zip): Likewise.
	(aarch64_evpc_ext): Likewise.
	(aarch64_evpc_rev): Likewise.
	(aarch64_evpc_dup): Test the encoding for a single duplicated element,
	instead of checking each element individually.  Return true without
	generating rtl if
	(aarch64_vectorize_vec_perm_const): Use all_from_input_p to test
	whether all selected elements come from the same input, instead of
	checking each element individually.  Remove calls to gen_rtx_REG,
	start_sequence and end_sequence and instead assert that no rtl is
	generated.

From-SVN: r256385