Mostly revert r254296

Mostly revert r254296

r254296 added support for (const ...) wrappers around vectors,
but in the end the agreement was to use a variable-length
encoding of CONST_VECTOR (and VECTOR_CST) instead.  This patch
therefore reverts the bits that are no longer needed.

The rtl.texi part isn't a full revert, since r254296 also updated the
documentation to mention unspecs in address calculations, and to relax
the requirement that the mode had to be Pmode.

2018-01-10  Richard Sandiford  <richard.sandiford@linaro.org>

gcc/
	* doc/rtl.texi: Remove documentation of (const ...) wrappers
	for vectors, as a partial revert of r254296.
	* rtl.h (const_vec_p): Delete.
	(const_vec_duplicate_p): Don't test for vector CONSTs.
	(unwrap_const_vec_duplicate, const_vec_series_p): Likewise.
	* expmed.c (make_tree): Likewise.

	Revert:
	* 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.

From-SVN: r256421