Include phis in SLP unrolling calculation

Include phis in SLP unrolling calculation

Without this we'd pick an unrolling factor based purely on longs,
ignoring the ints.  It's posssible that vect_get_smallest_scalar_type
should also handle shifts, but I think we'd still want this as a
belt-and-braces fix.

2017-09-18  Richard Sandiford  <richard.sandiford@linaro.org>

gcc/
	* tree-vect-slp.c (vect_record_max_nunits): New function,
	split out from...
	(vect_build_slp_tree_1): ...here.
	(vect_build_slp_tree_2): Call it for phis too.

gcc/testsuite/
	* gcc.dg/vect/slp-multitypes-13.c: New test.

From-SVN: r252933