Invoke vectorizable_live_operation in a consistent way

Invoke vectorizable_live_operation in a consistent way

vect_transform_stmt calls vectorizable_live_operation for
each live statement in an SLP node, but vect_analyze_stmt
only called it the once.  This patch makes vect_analyze_stmt
consistent with vect_transform_stmt, which should be a bit
more robust, and also means that a later patch can use
slp_index when deciding validity.

2017-09-18  Richard Sandiford  <richard.sandiford@linaro.org>
	    Alan Hayward  <alan.hayward@arm.com>
	    David Sherwood  <david.sherwood@arm.com>

gcc/
	* tree-vect-stmts.c (can_vectorize_live_stmts): New function,
	split out from...
	(vect_transform_stmt): ...here.
	(vect_analyze_stmt): Use it instead of calling
	vectorizable_live_operation directly.

Co-Authored-By: Alan Hayward <alan.hayward@arm.com>
Co-Authored-By: David Sherwood <david.sherwood@arm.com>
From-SVN: r252929