vect: Pattern-matched calls in reduction chains

vect: Pattern-matched calls in reduction chains

gcc.dg/pr56350.c started ICEing for SVE in GCC 10 because we
pattern-matched a division reduction:

      a /= 8;

into a signed shift with division semantics:

      ... = IFN_SDIV_POW2 (..., 3);

whereas the reduction code expected it still to be a gassign.

One fix would be to check for a reduction in the pattern matcher
(but current patterns don't generally do that).  Another would be
to fail gracefully for reductions involving calls.  Since we can't
vectorise the reduction either way, and probably have a better shot
with the shift form, this patch goes for the "fail gracefully" approach.

2020-01-28  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	* tree-vect-loop.c (vectorizable_reduction): Fail gracefully
	for reduction chains that (now) include a call.