This patch makes vectorizable_live_operation cope with variable-length
vectors.  For now we just handle cases in which we can tell at compile
time which vector contains the final result.

2018-01-03  Richard Sandiford  <richard.sandiford@linaro.org>
	    Alan Hayward  <alan.hayward@arm.com>
	    David Sherwood  <david.sherwood@arm.com>

gcc/
	* tree-vect-loop.c (vectorizable_live_operation): Treat the number
	of units as polynomial.  Punt if we can't tell at compile time
	which vector contains the final result.

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

From-SVN: r256135