[1/2] Add get_next_strinfo helper function

This patch just adds a helper function for getting the next strinfo
in a chain, since part 2 adds another place where we do that.

2017-05-16  Richard Sandiford  <richard.sandiford@linaro.org>

gcc/
	* tree-ssa-strlen.c (get_next_strinfo): New function.
	(get_stridx_plus_constant): Use it.
	(zero_length_string): Likewise.
	(adjust_related_strinfos): Likewise.
	(adjust_last_stmt): Likewise.

From-SVN: r248732