Add LOOP_VINFO_MAX_VECT_FACTOR

Epilogue vectorisation uses the vectorisation factor of the main loop
as the maximum vectorisation factor allowed for correctness.  That makes
sense as a conservatively correct value, since the chosen vectorisation
factor will be strictly less than that anyway.  However, once the VF
itself becomes variable, it's easier to carry across the original
maximum VF instead.

2017-09-14  Richard Sandiford  <richard.sandiford@linaro.org>
	    Alan Hayward  <alan.hayward@arm.com>
	    David Sherwood  <david.sherwood@arm.com>

gcc/
	* tree-vectorizer.h (_loop_vec_info): Add max_vectorization_factor.
	(LOOP_VINFO_MAX_VECT_FACTOR): New macro.
	(LOOP_VINFO_ORIG_VECT_FACTOR): Replace with...
	(LOOP_VINFO_ORIG_MAX_VECT_FACTOR): ...this new macro.
	* tree-vect-data-refs.c (vect_analyze_data_ref_dependences): Update
	accordingly.
	* tree-vect-loop.c (_loop_vec_info::_loop_vec_info): Initialize
	max_vectorization_factor.
	(vect_analyze_loop_2): Set LOOP_VINFO_MAX_VECT_FACTOR.

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

From-SVN: r252766