[AArch64] Generalise aarch64_simd_valid_immediate for SVE

[AArch64] Generalise aarch64_simd_valid_immediate for SVE

The current aarch64_simd_valid_immediate code predates the move
to the new CONST_VECTOR representation, so for variable-length SVE
it only handles duplicates of single elements, rather than duplicates
of repeating patterns.

This patch removes the restriction.  It means that the validity
of a duplicated constant depends only on the bit pattern, not on
the mode used to represent it.

The patch is needed by a later big-endian fix.

2018-02-01  Richard Sandiford  <richard.sandiford@linaro.org>

gcc/
	* config/aarch64/aarch64.c (aarch64_simd_valid_immediate): Handle
	all CONST_VECTOR_DUPLICATE_P vectors, not just those with a single
	duplicated element.

Reviewed-by: James Greenhalgh <james.greenhalgh@arm.com>
From-SVN: r257286