[6/n] PR85694: Add a vect_get_internal_def helper

[6/n] PR85694: Add a vect_get_internal_def helper

This patch adds a helper for pattern code that wants to find an
internal (vectorisable) definition of an SSA name.

A later patch will make more use of this, and alter the definition.

2018-06-20  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	* tree-vect-patterns.c (vect_get_internal_def): New function.
	(vect_recog_dot_prod_pattern, vect_recog_sad_pattern)
	(vect_recog_vector_vector_shift_pattern, check_bool_pattern)
	(search_type_for_mask_1): Use it.

From-SVN: r261789