Add a vect_worthwhile_without_simd_p helper routine

Add a vect_worthwhile_without_simd_p helper routine

The vectoriser sometimes considers lowering "vector" operations into N
scalar word operations.  This N needs to be fixed at compile time, so
the condition guarding it needs to change when variable-lengh vectors
are added.  This patch puts the condition into a helper routine so that
there's only one place to update.

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 (vect_min_worthwhile_factor): Delete.
	(vect_worthwhile_without_simd_p): Declare.
	* tree-vect-loop.c (vect_worthwhile_without_simd_p): New function.
	(vectorizable_reduction): Use it.
	* tree-vect-stmts.c (vectorizable_shift): Likewise.
	(vectorizable_operation): Likewise.

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@252765 138bc75d-0d04-0410-961f-82ee72b054a4