[02/46] Remove dead vectorizable_reduction code

[02/46] Remove dead vectorizable_reduction code

vectorizable_reduction has old code to cope with cases in which the
given statement belongs to a reduction group but isn't the first statement.
That can no longer happen, since all statements in the group go into the
same SLP node, and we only check the first statement in each node.

The point is to remove the only path through vectorizable_reduction
in which stmt and stmt_info refer to different statements.

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

gcc/
	* tree-vect-loop.c (vectorizable_reduction): Assert that the
	function is not called for second and subsequent members of
	a reduction group.

From-SVN: r263117