[09/46] Add vec_info::lookup_single_use

[09/46] Add vec_info::lookup_single_use

This patch adds a helper function for seeing whether there is a single
user of an SSA name, and whether that user has a stmt_vec_info.

2018-07-31  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	* tree-vectorizer.h (vec_info::lookup_single_use): Declare.
	* tree-vectorizer.c (vec_info::lookup_single_use): New function.
	* tree-vect-loop.c (vectorizable_reduction): Use it instead of
	a single_imm_use-based sequence.
	* tree-vect-stmts.c (supportable_widening_operation): Likewise.

From-SVN: r263124