Store VECTOR_CST_NELTS directly in tree_node

Store VECTOR_CST_NELTS directly in tree_node

Previously VECTOR_CST_NELTS (t) read the number of elements from
TYPE_VECTOR_SUBPARTS (TREE_TYPE (t)).  There were two ways of handling
this with variable TYPE_VECTOR_SUBPARTS: either forcibly convert the
number to a constant (which is doable) or store the number directly
in the VECTOR_CST.  The latter seemed better, since it involves less
pointer chasing and since the tree_node u field is otherwise unused
for VECTOR_CST.  It would still be easy to switch to the former in
future if we need to free up the field for someting else.

The patch also changes various bits of VECTOR_CST code to use
VECTOR_CST_NELTS instead of TYPE_VECTOR_SUBPARTS when iterating
over VECTOR_CST_ELTs.  Also, when the two are checked for equality,
the patch prefers to read VECTOR_CST_NELTS (which must be constant)
and check against TYPE_VECTOR_SUBPARTS, instead of the other way
around.

2017-09-14  Richard Sandiford  <richard.sandiford@linaro.org>
	    Alan Hayward  <alan.hayward@arm.com>
	    David Sherwood  <david.sherwood@arm.com>

gcc/
	* tree-core.h (tree_base::u): Add an "nelts" field.
	(tree_vector): Use VECTOR_CST_NELTS as the length.
	* tree.c (tree_size): Likewise.
	(make_vector): Initialize VECTOR_CST_NELTS.
	* tree.h (VECTOR_CST_NELTS): Use the u.nelts field.
	* cfgexpand.c (expand_debug_expr): Use VECTOR_CST_NELTS instead of
	TYPE_VECTOR_SUBPARTS.
	* expr.c (const_vector_mask_from_tree): Consistently use "units"
	as the number of units, setting it from VECTOR_CST_NELTS.
	(const_vector_from_tree): Likewise.
	* fold-const.c (negate_expr_p): Use VECTOR_CST_NELTS instead of
	TYPE_VECTOR_SUBPARTS for the number of elements in a VECTOR_CST.
	(fold_negate_expr_1): Likewise.
	(fold_convert_const): Likewise.
	(const_binop): Likewise.  Differentiate the number of output and
	input elements.
	(const_unop): Likewise.
	(fold_ternary_loc): Use VECTOR_CST_NELTS for the number of elements
	in a VECTOR_CST, asserting that it is the same as TYPE_VECTOR_SUBPARTS
	in cases that did the opposite.

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@252758 138bc75d-0d04-0410-961f-82ee72b054a4