Disallow EXTRACT_LAST_REDUCTION for reduction chains

Disallow EXTRACT_LAST_REDUCTION for reduction chains

gcc.dg/vect/vect-cond-reduc-5.c was ICEing for SVE because we
tried to use an extract-last reduction for a chain of COND_EXPRs.
Adding support for the chained case would be too invasive for stage 3
so this patch explicitly forbids it instead.  I've filed PR92884 for
the possible future work.

2019-12-10  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	* tree-vect-loop.c (vectorizable_reduction): Don't use
	EXTRACT_LAST_REDUCTION for chained reductions.

From-SVN: r279161