[2/3] [vect] Add widening add, subtract patterns

[2/3] [vect] Add widening add, subtract patterns

Add widening add, subtract patterns to tree-vect-patterns. Update the
widened code of patterns that detect PLUS_EXPR to also detect
WIDEN_PLUS_EXPR. These patterns take 2 vectors with N elements of size
S and perform an add/subtract on the elements, storing the results as N
elements of size 2*S (in 2 result vectors). This is implemented in the
aarch64 backend as addl,addl2 and subl,subl2 respectively. Add aarch64
tests for patterns.

gcc/ChangeLog:
	* doc/generic.texi: Document new widen_plus/minus_lo/hi tree codes.
	* doc/md.texi: Document new widenening add/subtract hi/lo optabs.
	* expr.c (expand_expr_real_2): Add widen_add, widen_subtract cases.
	* optabs-tree.c (optab_for_tree_code): Add case for widening optabs.
	* optabs.def (OPTAB_D): Define vectorized widen add, subtracts.
	* tree-cfg.c (verify_gimple_assign_binary): Add case for widening adds,
	subtracts.
	* tree-inline.c (estimate_operator_cost): Add case for widening adds,
	subtracts.
	* tree-vect-generic.c (expand_vector_operations_1): Add case for
	widening adds, subtracts
	* tree-vect-patterns.c (vect_recog_widen_add_pattern): New recog
	pattern.
	(vect_recog_widen_sub_pattern): New recog pattern.
	(vect_recog_average_pattern): Update widened add code.
	(vect_recog_average_pattern): Update widened add code.
	* tree-vect-stmts.c (vectorizable_conversion): Add case for widened add,
	subtract.
	(supportable_widening_operation): Add case for widened add, subtract.
	* tree.def
	(WIDEN_PLUS_EXPR): New tree code.
	(WIDEN_MINUS_EXPR): New tree code.
	(VEC_WIDEN_ADD_HI_EXPR): New tree code.
	(VEC_WIDEN_PLUS_LO_EXPR): New tree code.
	(VEC_WIDEN_MINUS_HI_EXPR): New tree code.
	(VEC_WIDEN_MINUS_LO_EXPR): New tree code.

gcc/testsuite/ChangeLog:

	* gcc.target/aarch64/vect-widen-add.c: New test.
	* gcc.target/aarch64/vect-widen-sub.c: New test.