This patch allows (const ...) wrappers to be used for rtx vector
constants, as an alternative to const_vector.  This is useful
for SVE, where the number of elements isn't known until runtime.

It could also be useful in future for fixed-length vectors, to
reduce the amount of memory needed to represent simple constants
with high element counts.  However, one nice thing about keeping
it restricted to variable-length vectors is that there is never
any need to handle combinations of (const ...) and CONST_VECTOR.

2017-11-01  Richard Sandiford  <richard.sandiford@linaro.org>
	    Alan Hayward  <alan.hayward@arm.com>
	    David Sherwood  <david.sherwood@arm.com>

gcc/
	* doc/rtl.texi (const): Update description of address constants.
	Say that vector constants are allowed too.
	* common.md (E, F): Use CONSTANT_P instead of checking for
	CONST_VECTOR.
	* emit-rtl.c (gen_lowpart_common): Use const_vec_p instead of
	checking for CONST_VECTOR.
	* expmed.c (make_tree): Use build_vector_from_val for a CONST
	VEC_DUPLICATE.
	* expr.c (expand_expr_real_2): Check for vector modes instead
	of checking for CONST_VECTOR.
	* rtl.h (const_vec_p): New function.
	(const_vec_duplicate_p): Check for a CONST VEC_DUPLICATE.
	(unwrap_const_vec_duplicate): Handle them here too.

Co-Authored-By: Alan Hayward <alan.hayward@arm.com>
Co-Authored-By: David Sherwood <david.sherwood@arm.com>

From-SVN: r254296